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.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"),
     },
   ];