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/header.json

@@ -5,7 +5,7 @@
   "searchYoutubeTip": "Search live on YouTube for this term (uses shared API quota). You can also press Enter.",
   "go": "Go",
   "goTip": "Search (or press Enter)",
-  "channelManager": "Channel manager",
+  "channelManager": "Channels",
   "usageStats": "Usage & stats",
   "scheduler": "Scheduler",
   "configuration": "Configuration",
@@ -15,7 +15,7 @@
     "my": "Mine",
     "myTip": "Only videos from your own subscriptions",
     "all": "Library",
-    "allTip": "Every video in the shared catalog"
+    "allTip": "Every video in the shared Library"
   },
   "account": {
     "admin": "Admin",
@@ -51,14 +51,14 @@
     "resume": "Resume sync"
   },
   "banner": {
-    "updated": "Updated to v{{version}} — see what's changed.",
+    "updated": "Updated to v{{version}} — see what's changed",
     "releaseNotes": "Release notes",
     "dismiss": "Dismiss"
   },
   "verify": {
-    "confirmed": "Email address confirmed.",
-    "confirmedNamed": "Email address confirmed — {{email}}.",
-    "invalid": "That verification link is no longer valid — it has expired or was already used.",
+    "confirmed": "Email address confirmed",
+    "confirmedNamed": "Email address confirmed — {{email}}",
+    "invalid": "That verification link is no longer valid — it has expired or was already used",
     "unreachable": "Couldn't reach the server to confirm that link, so it hasn't been used up. Try opening it again in a moment.",
     "resetSignedOut": "Password-reset links open in a signed-out window. Sign out here, or open the link in a private window, and it will pick up where it left off.",
     "reset": "The password this account registered with was cleared for security; set a new one before signing in."