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/VideoCard.tsx

@@ -22,7 +22,7 @@ import { navItemAttrs } from "../lib/listNav";
 import clsx from "clsx";
 import type { Video, VideoStatus } from "../lib/api";
 import { FEED_VIEW_SPEC, type FeedView } from "../lib/feedView";
-import { formatDate, formatDuration, formatViews, relativeTime } from "../lib/format";
+import { formatDate, formatDuration, formatNumber, formatViews, relativeTime } from "../lib/format";
 import { isPlainLeftClick } from "../lib/nav";
 import ChannelNameLink from "./ChannelNameLink";
 
@@ -423,7 +423,7 @@ function VideoCard({
   // on every line and the tooltip carries the figure regardless.
   const viewsCell = (withWord: boolean) =>
     video.view_count != null && (
-      <span title={`${video.view_count.toLocaleString()} ${t("card.views")}`}>
+      <span title={`${formatNumber(video.view_count)} ${t("card.views")}`}>
         {formatViews(video.view_count)}
         {withWord && ` ${t("card.views")}`}
       </span>