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/DebugRecorder.tsx

@@ -16,7 +16,7 @@ import {
   useStepCount,
   useStepPrompt,
 } from "../lib/debugRecorder";
-import { formatDuration } from "../lib/format";
+import { formatDateTimeSec, formatDuration } from "../lib/format";
 import { useLayer } from "../lib/layerStack";
 import { notify } from "../lib/notifications";
 import { qk } from "../lib/queryKeys";
@@ -372,7 +372,7 @@ const chipTone = (s: DebugSession): string => {
 
 /** The last sessions, newest first — reopening one opens its composer, with what was written. */
 function SessionHistory() {
-  const { t, i18n } = useTranslation();
+  const { t } = useTranslation();
   const q = useQuery({ queryKey: qk.debugSessions(), queryFn: api.debugSessions });
   const items = q.data?.items.filter((s) => s.ended_at) ?? [];
   if (q.isError && !q.data)
@@ -394,9 +394,7 @@ function SessionHistory() {
               key={s.id}
               className="flex flex-wrap items-center gap-x-3 gap-y-1 px-3 py-2 text-sm"
             >
-              <span className="tabular-nums">
-                {new Date(s.started_at).toLocaleString(i18n.language)}
-              </span>
+              <span className="tabular-nums">{formatDateTimeSec(s.started_at)}</span>
               <span className="min-w-0 flex-1 truncate text-muted">
                 {s.draft?.title || t(`settings.diagnostics.levels.${s.level}`)}
               </span>