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/NotificationsPanel.tsx

@@ -36,7 +36,7 @@ import {
   type VideoHiddenMeta,
   type VideoWatchedMeta,
 } from "../lib/notifications";
-import { channelYouTubeUrl, relativeFromMs, relativeTime } from "../lib/format";
+import { channelYouTubeUrl } from "../lib/format";
 import { Button } from "./ui/Button";
 import { IconButton } from "./ui/IconButton";
 import { LEVEL_STYLE } from "./ui/toastLevels";
@@ -57,6 +57,7 @@ import { type ActionMenuItem } from "./ui/ActionMenu";
 import { menuGroups } from "./ui/menuGroups";
 import { HOVER_REVEAL } from "./ui/reveal";
 import { useConfirm } from "./confirmContext";
+import { RelativeTime } from "./ui/RelativeTime";
 
 // The single notification center. "System" = durable, server-backed notifications (inbox
 // phase 1). "Activity" = the client-side transient events (action confirmations, errors, with
@@ -545,7 +546,7 @@ function NotificationRow({
       <div className="flex-1 min-w-0">
         <div className="flex items-baseline gap-2">
           <span className="font-medium text-sm">{title}</span>
-          <span className="text-2xs text-muted shrink-0">{relativeTime(n.created_at)}</span>
+          <RelativeTime at={n.created_at} className="text-2xs text-muted shrink-0" />
         </div>
         {body && <div className="text-sm text-muted mt-0.5">{body}</div>}
         {videos.length > 0 && (
@@ -634,7 +635,7 @@ function ClientActivityRow({
           )}
         </div>
         <div className="flex items-center gap-2 mt-0.5">
-          <span className="text-2xs text-muted">{relativeFromMs(n.ts)}</span>
+          <RelativeTime at={n.ts} className="text-2xs text-muted" />
           {needsAction && (
             <span className="text-3xs uppercase tracking-wide font-semibold text-accent">
               {t("notifications.actionNeeded")}