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/feed.json

@@ -12,16 +12,16 @@
     "maxAge_other": "Last {{count}} days"
   },
   "loading": "Loading Feed…",
-  "loadError": "Couldn't load the Feed.",
+  "loadError": "Couldn't load the Feed",
   "emptyTitle": "Your Feed is empty",
   "emptyBody": "Connect your YouTube account to import your subscriptions and build your Feed.",
   "setUp": "Set up my Feed",
-  "browseShared": "Or just browse the shared library",
+  "browseShared": "Or just browse the shared Library",
   "demoEmptyTitle": "Nothing here in the demo",
-  "demoEmptyBody": "The demo account has no subscriptions of its own — but the whole shared library is yours to explore, filter and sort.",
-  "browseLibrary": "Browse the library",
+  "demoEmptyBody": "The demo account has no subscriptions of its own — but the whole shared Library is yours to explore, filter and sort.",
+  "browseLibrary": "Browse the Library",
   "noMatches": "No videos match these filters.",
-  "channelScopeHint": "This channel isn't in your Feed — it may be hidden. Its videos are in the shared library.",
+  "channelScopeHint": "This channel isn't in your Feed — it may be hidden. Its videos are in the shared Library.",
   "viewInLibrary": "View in Library",
   "sortLabel": "Sort",
   "viewLabel": "View",
@@ -70,7 +70,7 @@
     "back": "Back to Feed",
     "searching": "Searching YouTube…",
     "noResults": "No YouTube videos found.",
-    "error": "YouTube search failed.",
+    "error": "Couldn't search YouTube",
     "loadMore": "Load more (uses quota)",
     "loadMoreFree": "Load more",
     "show": "Show",
@@ -78,6 +78,6 @@
     "clearNow": "Clear now",
     "cleared_one": "Cleared {{count}} result",
     "cleared_other": "Cleared {{count}} results",
-    "showHint": "How many results to fetch at a time — Load more pulls another batch of this size."
+    "showHint": "How many results to fetch at a time — “Load more” pulls another batch of this size."
   }
 }