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/Messages.tsx

@@ -8,7 +8,6 @@ import { useLiveQuery } from "../lib/useLiveQuery";
 import { Button } from "./ui/Button";
 import { IconButton } from "./ui/IconButton";
 import { LoadingState, StateMessage } from "./ui/QueryState";
-import { relativeTime } from "../lib/format";
 import { partnerPub, POLL_MS, SYSTEM_ID, useKeyState } from "../lib/messaging";
 import { useHistorySubview } from "../lib/history";
 import { openChat } from "../lib/chatDock";
@@ -22,6 +21,7 @@ import { Pressable } from "./ui/Pressable";
 import { Input } from "./ui/Input";
 import { SURFACE } from "./ui/surfaces";
 import { SectionLabel } from "./ui/Headings";
+import { RelativeTime } from "./ui/RelativeTime";
 
 type View =
   | "list"
@@ -177,9 +177,10 @@ function ConversationList({
                       >
                         {c.partner.name}
                       </span>
-                      <span className="ml-auto text-2xs text-muted shrink-0">
-                        {relativeTime(c.last_message.created_at)}
-                      </span>
+                      <RelativeTime
+                        at={c.last_message.created_at}
+                        className="ml-auto text-2xs text-muted shrink-0"
+                      />
                     </div>
                     <div className="flex items-center gap-2">
                       <span