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/common.json

@@ -1,6 +1,6 @@
 {
-  "privacyPolicy": "Privacy Policy",
-  "termsOfService": "Terms of Service",
+  "privacyPolicy": "Privacy policy",
+  "termsOfService": "Terms of service",
   "sourceCode": "Source code",
   "close": "Close",
   "cancel": "Cancel",
@@ -10,7 +10,7 @@
   "undo": "Undo",
   "redo": "Redo",
   "loading": "Loading…",
-  "somethingWrong": "Something went wrong.",
+  "somethingWrong": "Something went wrong",
   "itemCount_one": "{{formattedCount}} item",
   "itemCount_other": "{{formattedCount}} items",
   "accessRequestsTitle": "Access requests",
@@ -20,7 +20,7 @@
   "demoSharedNotice": "You're in the shared demo account. Everything you do here — playlists, hidden videos, settings — is communal and may be changed by other demo visitors at the same time.",
   "backToTop": "Back to top",
   "retry": "Retry",
-  "loadError": "Couldn't load this — check your connection and try again.",
+  "loadError": "Couldn't load this — check your connection and try again",
   "seeAll": "See all",
   "scrollLeft": "Scroll left",
   "scrollRight": "Scroll right",