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/ChatThread.tsx

@@ -8,12 +8,12 @@ import { useLiveQuery } from "../lib/useLiveQuery";
 import { useScrollFade } from "../lib/useScrollFade";
 import { IconButton } from "./ui/IconButton";
 import { LoadingState, StateMessage } from "./ui/QueryState";
-import { relativeTime } from "../lib/format";
 import { notifyActionError } from "../lib/actionErrors";
 import { partnerPub, POLL_MS, refreshPartnerPub, useKeyState } from "../lib/messaging";
 import * as e2ee from "../lib/e2ee";
 import KeyGate from "./KeyGate";
 import { Textarea } from "./ui/Input";
+import { RelativeTime } from "./ui/RelativeTime";
 
 // The message list + composer for one conversation, shared by the full Messages page and the
 // floating dock window. Handles client-side decryption and encrypt-on-send; the surrounding
@@ -194,9 +194,10 @@ export default function ChatThread({
                   }`}
                 >
                   {text}
-                  <div className={`text-3xs mt-1 ${mine ? "text-accent-fg/70" : "text-muted"}`}>
-                    {relativeTime(m.created_at)}
-                  </div>
+                  <RelativeTime
+                    at={m.created_at}
+                    className={`block text-3xs mt-1 ${mine ? "text-accent-fg/70" : "text-muted"}`}
+                  />
                 </div>
               </div>
             );