fix: numbers follow the UI language, not the browser's

`n.toLocaleString()` groups in the BROWSER's locale, so an English UI in a
Hungarian browser printed "1 234 567" while a Hungarian UI in an English
browser printed "1,234,567" — the number half of the date defect fixed in
618ef812. One `formatNumber` in lib/format.ts (a per-language cached
Intl.NumberFormat, like the relative-time formatters beside it) now serves
every grouped figure: Stats, Scheduler, the channel columns and the stats
popover, VideoCard's view tooltip, the ItemCount pill, and formatTotalHours.

ReleaseNotes' `toLocaleDateString(undefined, …)` was the one DATE site the
previous sweep missed; it moves onto formatDate.

The new cases assert both languages and that the two differ, instead of
comparing against the runtime's own default: that default is Hungarian on the
dev box and English on a CI image, so a one-language assertion would pass
under the old bug on half the machines. Verified in Chromium on :5173 with the
browser forced to hu-HU and the UI in English — Stats, Scheduler and the
channel table all keep comma grouping.

frontend/src/components/ChannelStatsPopover.dom.test.tsx

@@ -19,7 +19,7 @@ vi.mock("react-i18next", async (importOriginal) => {
 
 import ChannelStatsPopover from "./ChannelStatsPopover";
 import { type MyStatus } from "../lib/api";
-import { formatEta } from "../lib/format";
+import { formatEta, formatNumber } from "../lib/format";
 
 beforeAll(() => {
   class Stub {
@@ -74,7 +74,7 @@ describe("ChannelStatsPopover", () => {
     expect(screen.getByText("channels.stats.recentSynced")).toBeTruthy();
     expect(screen.getByText("3/10")).toBeTruthy(); // recent-synced ratio
     expect(screen.getByText("1/4")).toBeTruthy(); // full-history ratio
-    expect(screen.getByText((1234).toLocaleString())).toBeTruthy(); // my_videos, locale-agnostic
+    expect(screen.getByText(formatNumber(1234))).toBeTruthy(); // my_videos, locale-agnostic
   });
 
   it("shows the 'left' ETA row only while a backfill is pending, with the formatted ETA value", () => {

frontend/src/components/ChannelStatsPopover.tsx

@@ -11,7 +11,7 @@ import {
 } from "lucide-react";
 import { useTranslation } from "react-i18next";
 import { type MyStatus } from "../lib/api";
-import { formatEta } from "../lib/format";
+import { formatEta, formatNumber } from "../lib/format";
 import { PopoverSurface, useAnchoredPopover } from "./ui/AnchoredPopover";
 
 // The channel-manager catalog stats, collapsed from a crowded inline row into a compact "Stats" chip
@@ -63,13 +63,13 @@ export default function ChannelStatsPopover({ status }: { status: MyStatus }) {
     {
       icon: Film,
       label: t("channels.stats.myVideos"),
-      value: status.my_videos.toLocaleString(),
+      value: formatNumber(status.my_videos),
       hint: t("channels.stats.myVideosHint"),
     },
     {
       icon: Zap,
       label: t("channels.stats.quotaLeft"),
-      value: status.quota_remaining_today.toLocaleString(),
+      value: formatNumber(status.quota_remaining_today),
       hint: t("channels.stats.quotaLeftHint"),
     },
   ];

frontend/src/components/ReleaseNotes.tsx

@@ -1,13 +1,14 @@
 import { useTranslation } from "react-i18next";
 import { Bug, Sparkles } from "lucide-react";
 import { RELEASE_NOTES } from "../lib/releaseNotes";
+import { formatDate } from "../lib/format";
 import Modal from "./ui/Modal";
 
-function fmtDate(d: string): string {
-  const t = new Date(d);
-  return isNaN(t.getTime())
-    ? d
-    : t.toLocaleDateString(undefined, { year: "numeric", month: "short", day: "numeric" });
+// The release date in the UI language — `toLocaleDateString(undefined, …)` formats in the BROWSER's
+// locale, which put Hungarian-shaped dates in an English release-notes list. Unparseable input falls
+// back to the raw string (a hand-written note could carry anything).
+function fmtDate(d: string, lang: string): string {
+  return isNaN(new Date(d).getTime()) ? d : formatDate(d, lang);
 }
 
 // Public release notes (chores are intentionally omitted here). `highlight` rings the
@@ -19,7 +20,7 @@ export default function ReleaseNotes({
   onClose: () => void;
   highlight?: string;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
   // Ring the newest note if `highlight` isn't one of the listed versions — e.g. a Vite dev build
   // where the caller's FRONTEND_VERSION is "dev", or a VERSION bump whose note hasn't landed yet —
   // so the banner always rings *something* current instead of nothing.
@@ -36,7 +37,7 @@ export default function ReleaseNotes({
           >
             <div className="flex items-baseline gap-2 flex-wrap">
               <h3 className="text-base font-semibold">v{r.version}</h3>
-              <span className="text-xs text-muted">{fmtDate(r.date)}</span>
+              <span className="text-xs text-muted">{fmtDate(r.date, i18n.language)}</span>
               {r.sha && (
                 <span className="text-2xs font-mono text-muted bg-surface px-1.5 py-0.5 rounded-sm">
                   {r.sha}

frontend/src/components/Scheduler.tsx

@@ -22,7 +22,7 @@ import { useLiveQuery } from "../lib/useLiveQuery";
 import { Button } from "./ui/Button";
 import { LoadingState, StateMessage } from "./ui/QueryState";
 import { useConfirm } from "./ConfirmProvider";
-import { relativeTime } from "../lib/format";
+import { formatNumber, relativeTime } from "../lib/format";
 import { notify } from "../lib/notifications";
 import Tooltip from "./ui/Tooltip";
 import { notifyActionError } from "../lib/actionErrors";
@@ -103,9 +103,9 @@ function JobProgress({ p }: { p: SchedulerJob["progress"] }) {
         <span className="truncate">{phase}</span>
         <span className="tabular-nums shrink-0">
           {p?.total != null
-            ? `${p.current.toLocaleString()} / ${p.total.toLocaleString()}`
+            ? `${formatNumber(p.current)} / ${formatNumber(p.total)}`
             : p
-              ? p.current.toLocaleString()
+              ? formatNumber(p.current)
               : ""}
         </span>
       </div>
@@ -291,7 +291,7 @@ function MaintenanceSettings({
       </div>
       <div className="text-2xs text-muted mt-1.5">
         {t("scheduler.maintenance.batchNote", {
-          default: m.revalidate_batch_default.toLocaleString(),
+          default: formatNumber(m.revalidate_batch_default),
         })}
       </div>
     </div>
@@ -540,31 +540,31 @@ export default function Scheduler() {
             icon={Database}
             label={t("scheduler.queue.recentPending")}
             hint={t("scheduler.queue.recentPendingHint")}
-            value={data.queue.channels_recent_pending.toLocaleString()}
+            value={formatNumber(data.queue.channels_recent_pending)}
           />
           <Stat
             icon={Database}
             label={t("scheduler.queue.deepPending")}
             hint={t("scheduler.queue.deepPendingHint")}
-            value={data.queue.channels_deep_pending.toLocaleString()}
+            value={formatNumber(data.queue.channels_deep_pending)}
           />
           <Stat
             icon={Clock}
             label={t("scheduler.queue.enrichPending")}
             hint={t("scheduler.queue.enrichPendingHint")}
-            value={data.queue.videos_pending_enrich.toLocaleString()}
+            value={formatNumber(data.queue.videos_pending_enrich)}
           />
           <Stat
             icon={Clock}
             label={t("scheduler.queue.shortsPending")}
             hint={t("scheduler.queue.shortsPendingHint")}
-            value={data.queue.videos_pending_shorts.toLocaleString()}
+            value={formatNumber(data.queue.videos_pending_shorts)}
           />
           <Stat
             icon={RadioTower}
             label={t("scheduler.queue.liveRefresh")}
             hint={t("scheduler.queue.liveRefreshHint")}
-            value={data.queue.videos_live_refresh.toLocaleString()}
+            value={formatNumber(data.queue.videos_live_refresh)}
           />
         </div>
       </div>
@@ -579,15 +579,15 @@ export default function Scheduler() {
             <Gauge className="w-4 h-4 text-muted shrink-0" />
             <span>
               {t("scheduler.quotaUsed", {
-                used: data.quota.used_today.toLocaleString(),
-                budget: data.quota.daily_budget.toLocaleString(),
+                used: formatNumber(data.quota.used_today),
+                budget: formatNumber(data.quota.daily_budget),
               })}
             </span>
             <span className="flex-1" />
             <span className="text-muted text-xs">
               {t("scheduler.quotaRemaining", {
                 count: data.quota.remaining_today,
-                formatted: data.quota.remaining_today.toLocaleString(),
+                formatted: formatNumber(data.quota.remaining_today),
               })}
             </span>
           </div>

frontend/src/components/Stats.tsx

@@ -4,7 +4,7 @@ import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
 import { History, Pause, Play, RefreshCw } from "lucide-react";
 import { api, type AdminQuotaRow, type Me } from "../lib/api";
-import { formatEta, quotaActionLabel } from "../lib/format";
+import { formatEta, formatNumber, quotaActionLabel } from "../lib/format";
 import { notify } from "../lib/notifications";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import { useWizard } from "./WizardProvider";
@@ -115,10 +115,10 @@ function Overview({ me }: { me: Me }) {
               </SettingRow>
             )}
             <SettingRow label={t("stats.sync.myVideos")} hint={t("stats.sync.myVideosHint")}>
-              {s.my_videos.toLocaleString()}
+              {formatNumber(s.my_videos)}
             </SettingRow>
             <SettingRow label={t("stats.sync.quotaLeft")} hint={t("stats.sync.quotaLeftHint")}>
-              {s.quota_remaining_today.toLocaleString()}
+              {formatNumber(s.quota_remaining_today)}
             </SettingRow>
           </div>
         ) : status.isError ? (
@@ -135,16 +135,16 @@ function Overview({ me }: { me: Me }) {
           <>
             <div className="space-y-1.5 text-sm">
               <SettingRow label={t("stats.sync.today")} hint={t("stats.sync.todayHint")}>
-                {usage.data.today.toLocaleString()}
+                {formatNumber(usage.data.today)}
               </SettingRow>
               <SettingRow label={t("stats.sync.last7d")}>
-                {usage.data.last_7d.toLocaleString()}
+                {formatNumber(usage.data.last_7d)}
               </SettingRow>
               <SettingRow label={t("stats.sync.last30d")}>
-                {usage.data.last_30d.toLocaleString()}
+                {formatNumber(usage.data.last_30d)}
               </SettingRow>
               <SettingRow label={t("stats.sync.allTime")}>
-                {usage.data.all_time.toLocaleString()}
+                {formatNumber(usage.data.all_time)}
               </SettingRow>
             </div>
             {Object.keys(usage.data.by_action).length > 0 && (
@@ -155,7 +155,7 @@ function Overview({ me }: { me: Me }) {
                   .map(([action, units]) => (
                     <div key={action} className="flex justify-between gap-2">
                       <span>{quotaActionLabel(action)}</span>
-                      <span className="tabular-nums">{units.toLocaleString()}</span>
+                      <span className="tabular-nums">{formatNumber(units)}</span>
                     </div>
                   ))}
               </div>
@@ -274,7 +274,7 @@ function SystemStats() {
             <div className="text-xs text-muted mb-2">
               {t("stats.dailyTotal", {
                 count: data.range_days,
-                units: grandTotal.toLocaleString(),
+                units: formatNumber(grandTotal),
               })}
             </div>
             {data.daily.length === 0 ? (
@@ -285,7 +285,7 @@ function SystemStats() {
                   <div
                     key={d.day}
                     className="flex-1 h-full flex items-end bg-card/50 rounded-t"
-                    title={t("stats.dailyTooltip", { day: d.day, units: d.total.toLocaleString() })}
+                    title={t("stats.dailyTooltip", { day: d.day, units: formatNumber(d.total) })}
                   >
                     <div
                       className="w-full bg-accent hover:opacity-80 rounded-t transition"
@@ -328,7 +328,7 @@ function UserRow({ row, max }: { row: AdminQuotaRow; max: number }) {
           {isSystem ? t("stats.system") : row.email}
         </span>
         <span className="text-sm font-semibold tabular-nums shrink-0">
-          {row.total.toLocaleString()}
+          {formatNumber(row.total)}
         </span>
       </button>
       {/* Proportion bar */}
@@ -343,7 +343,7 @@ function UserRow({ row, max }: { row: AdminQuotaRow; max: number }) {
           {actions.map(([action, units]) => (
             <div key={action} className="flex items-center justify-between gap-2">
               <span>{quotaActionLabel(action)}</span>
-              <span className="tabular-nums">{units.toLocaleString()}</span>
+              <span className="tabular-nums">{formatNumber(units)}</span>
             </div>
           ))}
         </div>

frontend/src/components/VideoCard.tsx

@@ -22,7 +22,7 @@ import { navItemAttrs } from "../lib/listNav";
 import clsx from "clsx";
 import type { Video, VideoStatus } from "../lib/api";
 import { FEED_VIEW_SPEC, type FeedView } from "../lib/feedView";
-import { formatDate, formatDuration, formatViews, relativeTime } from "../lib/format";
+import { formatDate, formatDuration, formatNumber, formatViews, relativeTime } from "../lib/format";
 import { isPlainLeftClick } from "../lib/nav";
 import ChannelNameLink from "./ChannelNameLink";
 
@@ -423,7 +423,7 @@ function VideoCard({
   // on every line and the tooltip carries the figure regardless.
   const viewsCell = (withWord: boolean) =>
     video.view_count != null && (
-      <span title={`${video.view_count.toLocaleString()} ${t("card.views")}`}>
+      <span title={`${formatNumber(video.view_count)} ${t("card.views")}`}>
         {formatViews(video.view_count)}
         {withWord && ` ${t("card.views")}`}
       </span>

frontend/src/components/channelColumns.tsx

@@ -3,7 +3,7 @@ import type { TFunction } from "i18next";
 import { useTranslation } from "react-i18next";
 import { Check, Eye, EyeOff, History, UserMinus } from "lucide-react";
 import { NO_TAG_SENTINEL, type ManagedChannel } from "../lib/api";
-import { formatCountOrDash, formatTotalHours, relativeTime } from "../lib/format";
+import { formatCountOrDash, formatNumber, formatTotalHours, relativeTime } from "../lib/format";
 import type { Column } from "./ui/DataTable";
 import Tooltip from "./ui/Tooltip";
 import { PriorityStepper } from "./ui/PriorityStepper";
@@ -66,7 +66,7 @@ function StoredCell({ c }: { c: ManagedChannel }) {
           />
         </Tooltip>
       )}
-      {c.stored_videos.toLocaleString()}
+      {formatNumber(c.stored_videos)}
     </span>
   );
 }

frontend/src/components/ui/ItemCount.dom.test.tsx

@@ -8,11 +8,15 @@ import { render, screen, cleanup } from "@testing-library/react";
 import "../../i18n"; // side-effect: initialise the shared i18next instance
 import i18n from "i18next";
 import ItemCount from "./ItemCount";
+import { formatNumber } from "../../lib/format";
 
 beforeAll(async () => {
   await i18n.changeLanguage("en");
 });
-afterEach(cleanup);
+afterEach(async () => {
+  cleanup();
+  await i18n.changeLanguage("en"); // a case that switches language must not leak it to the next
+});
 
 const text = () => screen.getByTestId("item-count").textContent;
 
@@ -24,17 +28,31 @@ describe("ItemCount", () => {
 
   it("pluralises and thousands-separates a normal count", () => {
     render(<ItemCount count={192045} />);
-    // Separators come from toLocaleString (runtime-locale dependent) — compare against it, not a fixed
-    // en-US string, so the test asserts ItemCount's formatting path rather than the CI locale.
-    expect(text()).toBe(`${(192045).toLocaleString()} items`);
+    // Separators come from the shared formatNumber — compare against it, not a fixed en-US string,
+    // so the test asserts ItemCount's formatting path rather than the CI locale.
+    expect(text()).toBe(`${formatNumber(192045)} items`);
+  });
+
+  // The pill used `count.toLocaleString()`, which groups in the BROWSER's locale: a Hungarian UI on
+  // an English browser showed "192,045" instead of "192 045". The title carries the pure number (no
+  // plural string), and both languages are asserted — comparing against the runtime's own default
+  // would pass under the old bug on a machine whose locale happens to match the UI language.
+  it("groups in the UI language, not the browser's", async () => {
+    const title = () => screen.getByTestId("item-count").getAttribute("title");
+    render(<ItemCount count={192045} />);
+    const asEnglish = title();
+    cleanup();
+    await i18n.changeLanguage("hu");
+    render(<ItemCount count={192045} />);
+    expect(asEnglish).toBe(new Intl.NumberFormat("en").format(192045));
+    expect(title()).toBe(new Intl.NumberFormat("hu").format(192045));
+    expect(title()).not.toBe(asEnglish);
   });
 
   it("collapses a count of a million or more to 1M+, with the exact figure in the tooltip", () => {
     render(<ItemCount count={1_000_000} />);
     expect(text()).toBe("1M+ items"); // the "1M+" collapse is locale-independent
-    expect(screen.getByTestId("item-count").getAttribute("title")).toBe(
-      (1_000_000).toLocaleString(),
-    );
+    expect(screen.getByTestId("item-count").getAttribute("title")).toBe(formatNumber(1_000_000));
   });
 
   it("shows the icon-only fixed box (empty label) while the count is undefined", () => {

frontend/src/components/ui/ItemCount.tsx

@@ -1,5 +1,6 @@
 import { useTranslation } from "react-i18next";
 import { Layers } from "lucide-react";
+import { formatNumber } from "../../lib/format";
 
 // At a million and above we stop printing the exact figure — six digits is the readable ceiling, so
 // the pill shows "1M+" and the tooltip carries the precise number.
@@ -20,12 +21,12 @@ export default function ItemCount({ count }: { count?: number }) {
       ? ""
       : t("common.itemCount", {
           count,
-          formattedCount: count >= OVERFLOW_AT ? "1M+" : count.toLocaleString(),
+          formattedCount: count >= OVERFLOW_AT ? "1M+" : formatNumber(count),
         });
   return (
     <span
       data-testid="item-count"
-      title={count?.toLocaleString()}
+      title={count == null ? undefined : formatNumber(count)}
       className="glass-card inline-flex shrink-0 items-center justify-center gap-1.5 rounded-full border border-border px-3 py-1 text-sm text-muted min-w-[9rem] whitespace-nowrap"
     >
       <Layers className="h-3.5 w-3.5 shrink-0 text-muted" aria-hidden="true" />

frontend/src/lib/format.test.ts

@@ -1,5 +1,12 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
-import { RELATIVE_UNITS, formatRuntime, relativeTime, relativeFromMs } from "./format";
+import {
+  RELATIVE_UNITS,
+  formatNumber,
+  formatRuntime,
+  formatTotalHours,
+  relativeTime,
+  relativeFromMs,
+} from "./format";
 import en from "../i18n/locales/en/time.json";
 import hu from "../i18n/locales/hu/time.json";
 
@@ -114,6 +121,48 @@ describe("relativeFromMs", () => {
 // implementation: 45s floored to "0m", so a playlist of Shorts reported no runtime while the rail
 // beside it read "0:45". The table pins every tier boundary, so the sub-minute branch cannot be
 // dropped again as "dead code" — it is the one branch a whole-playlist figure reaches.
+// A bare `n.toLocaleString()` groups in the BROWSER's locale, so an English UI in a Hungarian browser
+// printed "1 234 567" (and a Hungarian UI in an English browser "1,234,567"). The case below asserts
+// BOTH languages and that the two differ, rather than comparing against the runtime's own default:
+// the default is whatever locale the machine running the suite has (Hungarian on the dev box, English
+// on a CI image), so a one-language assertion passes under the old bug on half the machines.
+describe("formatNumber", () => {
+  afterEach(() => {
+    i18nStub.language = "en";
+  });
+
+  it("groups in the UI language, not in the runtime's", () => {
+    const asEnglish = formatNumber(1234567);
+    i18nStub.language = "hu";
+    const asHungarian = formatNumber(1234567);
+    expect(asEnglish).toBe(new Intl.NumberFormat("en").format(1234567));
+    expect(asHungarian).toBe(new Intl.NumberFormat("hu").format(1234567));
+    // The load-bearing half: under `toLocaleString()` both come out in the runtime's one locale.
+    expect(asEnglish).not.toBe(asHungarian);
+  });
+
+  it("follows a language switch at runtime (the per-language cache is keyed, not frozen)", () => {
+    expect(formatNumber(1000)).toBe(new Intl.NumberFormat("en").format(1000));
+    i18nStub.language = "hu";
+    expect(formatNumber(1000)).toBe(new Intl.NumberFormat("hu").format(1000));
+    i18nStub.language = "en";
+    expect(formatNumber(1000)).toBe(new Intl.NumberFormat("en").format(1000));
+  });
+
+  it("renders nothing for a value it cannot read, so a caller's dash fallback covers it", () => {
+    expect(formatNumber(null)).toBe("");
+    expect(formatNumber(undefined)).toBe("");
+    expect(formatNumber(NaN)).toBe("");
+    expect(formatNumber(Infinity)).toBe("");
+    expect(formatNumber(0)).toBe("0"); // a real zero is a value, not a missing one
+  });
+
+  it("carries the UI language into the channel total-hours label", () => {
+    i18nStub.language = "hu";
+    expect(formatTotalHours(12345 * 3600)).toBe(`${new Intl.NumberFormat("hu").format(12345)} h`);
+  });
+});
+
 describe("formatRuntime", () => {
   const CASES: [number | null | undefined, string][] = [
     [null, ""],

frontend/src/lib/format.ts

@@ -132,7 +132,28 @@ export function formatEta(seconds: number): string {
 export function formatTotalHours(sec: number): string {
   if (!sec) return "—";
   const h = Math.round(sec / 3600);
-  return h >= 1 ? `${h.toLocaleString()} h` : `${Math.max(1, Math.round(sec / 60))} m`;
+  return h >= 1 ? `${formatNumber(h)} h` : `${Math.max(1, Math.round(sec / 60))} m`;
+}
+
+// One grouped-number formatter per language, built lazily — the same reason as the relative-time
+// formatters above: the stats tables and the feed format a number per row, and constructing an Intl
+// formatter is the expensive part. Keyed by language because the user can switch it at runtime.
+const numberFormatters = new Map<string, Intl.NumberFormat>();
+
+/** A grouped number in the UI language ("1,234,567" in English, "1 234 567" in Hungarian). A bare
+ * `n.toLocaleString()` groups in the BROWSER's locale instead, so an English UI in a Hungarian
+ * browser showed space-grouped figures and vice versa — the number half of the same defect
+ * `formatDateTime` fixes for dates. Empty string for a missing/non-finite value, so a caller's
+ * `|| "—"` fallback covers it like a missing one. */
+export function formatNumber(n: number | null | undefined): string {
+  if (n == null || !Number.isFinite(n)) return "";
+  const lang = i18n.language;
+  let formatter = numberFormatters.get(lang);
+  if (!formatter) {
+    formatter = new Intl.NumberFormat(lang);
+    numberFormatters.set(lang, formatter);
+  }
+  return formatter.format(n);
 }
 
 export function formatViews(n: number | null): string {