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/i18n/locales/hu/common.json

@@ -10,7 +10,7 @@
   "undo": "Visszavonás",
   "redo": "Újra",
   "loading": "Betöltés…",
-  "somethingWrong": "Hiba történt.",
+  "somethingWrong": "Hiba történt",
   "itemCount_one": "{{formattedCount}} elem",
   "itemCount_other": "{{formattedCount}} elem",
   "accessRequestsTitle": "Hozzáférési kérések",
@@ -20,7 +20,7 @@
   "demoSharedNotice": "Egy közös demo fiókban vagy. Minden, amit itt csinálsz — lejátszási listák, elrejtett videók, beállítások — közös, és más demo látogatók egyszerre módosíthatják.",
   "backToTop": "Vissza a tetejére",
   "retry": "Újra",
-  "loadError": "Nem sikerült betölteni — ellenőrizd a kapcsolatot, és próbáld újra.",
+  "loadError": "Nem sikerült betölteni — ellenőrizd a kapcsolatot, és próbáld újra",
   "seeAll": "Összes",
   "scrollLeft": "Görgetés balra",
   "scrollRight": "Görgetés jobbra",