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/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>