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/AddToPlaylist.tsx

@@ -154,7 +154,9 @@ function MembershipPanel({
         {/* A React portal still propagates events up the REACT tree, so a click inside this panel
             would otherwise reach the feed card that hosts the trigger and start playback. */}
         <div onClick={(e) => e.stopPropagation()}>
-          <div className="text-xs text-muted px-1.5 pb-1.5">{t("playlists.addToPlaylist")}</div>
+          <div className="text-xs text-muted px-1.5 pb-1.5">
+            {t("playlists.addToPlaylistTitle")}
+          </div>
           <div
             ref={fade.ref}
             style={fade.style}