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", () => {