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/en/inbox.json

@@ -8,13 +8,13 @@
   "markRead": "Mark read",
   "dismiss": "Dismiss",
   "rowMenu": "Notification actions",
-  "menu": "Notification center actions",
+  "menu": "Notifications actions",
   "sectionSystem": "System",
   "sectionActivity": "Activity",
   "andMore": "and {{count}} more",
   "openScheduler": "Open Scheduler",
   "openDownloads": "Open Downloads",
-  "revertFailed": "Couldn't undo that — please try again.",
+  "revertFailed": "Couldn't undo that — please try again",
   "downloadDone": {
     "title": "Download ready"
   },
@@ -28,11 +28,11 @@
   },
   "jobDone": {
     "titleOk": "{{job}} finished",
-    "titleError": "{{job}} failed"
+    "titleError": "Couldn't run {{job}}"
   },
   "emailUndelivered": {
     "title": "Email is not going out",
     "notConfigured": "SMTP is not configured, so nothing this instance sends is being delivered — verification links, password resets and access requests alike. One of them was “{{subject}}”. Set it up under Configuration → Email / SMTP.",
-    "sendFailed": "Sending failed, so messages are not being delivered. One of them was “{{subject}}”. Check Configuration → Email / SMTP and send a test; the server log has the reason."
+    "sendFailed": "Couldn't send email, so messages are not being delivered. One of them was “{{subject}}”. Check Configuration → Email / SMTP and send a test; the server log has the reason."
   }
 }