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/ui/RelativeTime.tsx

@@ -0,0 +1,32 @@
+import { formatDateTime, formatDateTimeSec, relativeFromMs } from "../../lib/format";
+
+// docs/DESIGN.md "Copy": a relative time ("3d ago") says how recent something is in a list, and the
+// exact moment is one hover away — so the pair travels together here instead of every list
+// remembering (or forgetting) its own `title`. `seconds` is the log form (the audit). An unreadable
+// or missing moment renders `empty` ("—" in a table cell) or nothing.
+export function RelativeTime({
+  at,
+  seconds = false,
+  empty = "",
+  className,
+}: {
+  /** An ISO timestamp, or epoch millis (a client-side notification). */
+  at: string | number | null | undefined;
+  seconds?: boolean;
+  empty?: string;
+  className?: string;
+}) {
+  const ms = typeof at === "number" ? at : at ? new Date(at).getTime() : NaN;
+  const text = Number.isFinite(ms) ? relativeFromMs(ms) : "";
+  if (!text) return empty ? <span className={className}>{empty}</span> : null;
+  const iso = new Date(ms).toISOString();
+  return (
+    <time
+      dateTime={iso}
+      title={seconds ? formatDateTimeSec(iso) : formatDateTime(iso)}
+      className={className}
+    >
+      {text}
+    </time>
+  );
+}