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/PlexPlayer.tsx

@@ -84,6 +84,7 @@ import { Button } from "./ui/Button";
 import { IconButton } from "./ui/IconButton";
 import { Pressable } from "./ui/Pressable";
 import { Slider } from "./ui/NativeControls";
+import { formatUnit } from "../lib/format";
 
 // The rich info overlay (poster/cast/ratings) reuses the same component as the card's info page.
 const PlexInfo = lazy(() => import("./PlexInfo"));
@@ -865,7 +866,7 @@ export default function PlexPlayer({ itemId, onClose, queue, fromStart = false }
         seekTo(playhead(absRef, loadStartRef) + zone * TAP_SKIP_S);
         flashOsd(
           t(zone < 0 ? "plex.player.tapBack" : "plex.player.tapForward", {
-            n: skips * TAP_SKIP_S,
+            d: formatUnit(skips * TAP_SKIP_S, "second"),
           }),
         );
         return;