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.controls.dom.test.tsx

@@ -14,7 +14,8 @@ vi.mock("react-i18next", async (importOriginal) => {
   return {
     ...actual,
     useTranslation: () => ({
-      t: (key: string, o?: { n?: number }) => (o?.n != null ? `${key}:${o.n}` : key),
+      t: (key: string, o?: { n?: number; d?: string }) =>
+        o?.d != null ? `${key}:${o.d}` : o?.n != null ? `${key}:${o.n}` : key,
       i18n: { language: "en" },
     }),
   };
@@ -223,11 +224,11 @@ describe("a double-tap on a side third skips 10 s (C5)", () => {
     pressPicture("touch", 800);
     expect(bandHidden()).toBe(false); // …which the second takes back
     await waitFor(() => expect(lastStart(session)).toEqual(["A", 1510]));
-    expect(screen.getByText("plex.player.tapForward:10")).toBeTruthy();
+    expect(screen.getByText("plex.player.tapForward:10 sec")).toBeTruthy();
 
     pressPicture("touch", 800);
     await waitFor(() => expect(lastStart(session)).toEqual(["A", 1520]));
-    expect(screen.getByText("plex.player.tapForward:20")).toBeTruthy();
+    expect(screen.getByText("plex.player.tapForward:20 sec")).toBeTruthy();
 
     pressPicture("touch", 50); // the other side, a fresh first tap: a toggle, no skip
     expect(lastStart(session)).toEqual(["A", 1520]);
@@ -249,7 +250,7 @@ describe("a double-tap on a side third skips 10 s (C5)", () => {
     pressPicture("touch", 100);
     pressPicture("touch", 100);
     await waitFor(() => expect(lastStart(session)).toEqual(["A", 1490]));
-    expect(screen.getByText("plex.player.tapBack:10")).toBeTruthy();
+    expect(screen.getByText("plex.player.tapBack:10 sec")).toBeTruthy();
   });
 });