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/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, ""],