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/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>
   );
 }