feat(design): the Copy rule — names, verbs, dates and units (E28 S3g)

The owner's picks (all the recommended options) become DESIGN.md's Copy
chapter and a sweep of both languages (742 values, no key drift):

- One name per feature: the rail's label, which may shorten it (Alerts
  -> Notifications, Fiókok -> Felhasználók, Download Center -> Downloads,
  Channel manager -> Channels); one "sync from YouTube" verb.
- Four verbs, four meanings (Delete / Remove / Dismiss / Discard, and
  their Hungarian pairs), checked against what each action does.
- Sentence case, the app's places capitalised; "…" on a command that
  asks first (a popover or form title got its own key without it);
  one-sentence messages without a stop or a tick, "Couldn't …"; curly
  quotes.
- Dates only from lib/format.ts, in the UI language: formatDateTime
  names the month and stops at the minute (it printed "9/27/2026,
  8:30:14 AM"), formatDateTimeSec for the log, the audit and the
  recorder; ui/RelativeTime pairs "3d ago" with the exact minute on
  hover. local/date-format refuses toLocale*String and Intl date
  formatters elsewhere.
- Units from Intl, short (formatUnit): runtimes, totals, the Scheduler
  countdown, the stats range, the player's tap skip. "2h 13m" was
  English in both languages.
- src/i18n/copyRule.test.ts holds the machine-readable part (quotes,
  "...", ticks, bare unit letters).

frontend/src/components/ChannelPage.tsx

@@ -33,7 +33,7 @@ import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import { notifyActionError } from "../lib/actionErrors";
 import { api, DEFAULT_TAG_MODES, type FeedFilters, type Me, type ChannelDetail } from "../lib/api";
 import type { FeedView } from "../lib/feedView";
-import { channelYouTubeUrl, formatViews } from "../lib/format";
+import { channelYouTubeUrl, formatDate, formatViews } from "../lib/format";
 import { ChannelTagsControl } from "./ChannelTags";
 import { useChannelTagActions } from "../lib/useChannelTagActions";
 import { Button } from "./ui/Button";
@@ -285,12 +285,7 @@ export default function ChannelPage({
 
   const name = ch?.title ?? initialName ?? channelId;
   const ytUrl = channelYouTubeUrl(channelId, ch?.handle);
-  const joined = ch?.published_at
-    ? new Date(ch.published_at).toLocaleDateString(i18n.language, {
-        year: "numeric",
-        month: "short",
-      })
-    : null;
+  const joined = ch?.published_at ? formatDate(ch.published_at, i18n.language) : null;
   // Handle + stats on one compact meta line under the name (saves the separate stats row).
   const metaParts = [
     ch?.handle ? `@${ch.handle.replace(/^@/, "")}` : null,