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/VideoCard.tsx

@@ -35,6 +35,7 @@ import {
 } from "./ui/mediaCardStyles";
 import { MediaCard, MediaProgress } from "./ui/MediaCard";
 import { MediaResumeButtons } from "./MediaResumeButtons";
+import { RelativeTime } from "./ui/RelativeTime";
 
 /** The "…" that opens the video's action list, and whether its menu is the open one. */
 type More = { props: Record<string, unknown>; open: boolean };
@@ -700,17 +701,7 @@ function VideoCard({
               {/* The small card keeps the relative time and moves the date into its tooltip: at a
                 shelf's width the full pair wrapped to a third meta line, and the tallest card sets
                 the whole shelf's height (every card fills its slot). */}
-              {spec.dense ? (
-                <span
-                  title={
-                    video.published_at ? formatDate(video.published_at, i18n.language) : undefined
-                  }
-                >
-                  {relativeTime(video.published_at)}
-                </span>
-              ) : (
-                published
-              )}
+              {spec.dense ? <RelativeTime at={video.published_at} /> : published}
             </div>
           </>
         )