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/setup.json

@@ -8,15 +8,15 @@
     "desc": "This is the first account and an administrator. You'll sign in with this email and password once setup is done.",
     "email": "Email address",
     "password": "Password (min. {{n}} characters)",
-    "invalidEmail": "Enter a valid email address.",
-    "shortPassword": "Password must be at least {{n}} characters."
+    "invalidEmail": "Enter a valid email address",
+    "shortPassword": "Password must be at least {{n}} characters"
   },
   "google": {
     "title": "Sign in with Google (optional)",
     "desc": "Paste your Google OAuth client ID and secret to enable Google sign-in and YouTube access. Leave both empty to skip — email+password works without it.",
     "clientId": "Google client ID",
     "clientSecret": "Google client secret",
-    "bothRequired": "Enter both the client ID and secret, or leave both empty to skip.",
+    "bothRequired": "Enter both the client ID and secret, or leave both empty to skip",
     "skipHint": "Leave empty to set this up later from the admin Configuration page."
   },
   "smtp": {
@@ -29,8 +29,8 @@
     "password": "SMTP password",
     "testTo": "Send a test email to…",
     "test": "Send test",
-    "testOk": "Test email sent — check the inbox.",
-    "testFail": "Couldn't send — check the settings.",
+    "testOk": "Test email sent — check the inbox",
+    "testFail": "Couldn't send — check the settings",
     "skipHint": "Leave empty to set this up later from the admin Configuration page."
   },
   "finish": {
@@ -46,5 +46,5 @@
     "title": "Setup link needed",
     "body": "Open the setup wizard using the link printed in the container logs at first start (it looks like …/setup?token=…). It's required so only the operator can configure this instance."
   },
-  "genericError": "Something went wrong — please try again."
+  "genericError": "Something went wrong — please try again"
 }