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/eslint.config.js

@@ -82,6 +82,37 @@ const focusSafeDisabled = {
     };
   },
 };
+// E28 S3g (docs/DESIGN.md "Copy"): the app shows a date in three forms plus the relative one, all
+// built in lib/format.ts in the UI language. A bare `toLocaleString()` formats in the BROWSER's locale
+// and as numbers with seconds; seven call sites had drifted into five shapes. Outside lib/format.ts
+// a `toLocale*String` call or an `Intl.DateTimeFormat` / `Intl.RelativeTimeFormat` is refused (a
+// number goes through formatNumber, which the same rule covers).
+const dateFormat = {
+  meta: {
+    type: "problem",
+    messages: {
+      date: "Format dates and numbers through lib/format.ts (formatDate, formatDateTime, formatDateTimeSec, relativeTime, formatNumber), in the UI language.",
+    },
+  },
+  create(context) {
+    const path = context.filename.split("\\").join("/");
+    // lib/format.ts is where the forms live; a test computes its expectations; the Plex player's
+    // on-screen clock (lib/plexPlayerPrefs.ts) is a clock face with its own 12/24h switch, not a date.
+    if (/\/src\/lib\/(format|plexPlayerPrefs)\.ts$|\.test\.tsx?$/.test(path)) return {};
+    return {
+      "CallExpression > MemberExpression.callee[property.name=/^toLocale(Date|Time)?String$/]"(
+        node,
+      ) {
+        context.report({ node, messageId: "date" });
+      },
+      "MemberExpression[object.name='Intl'][property.name=/^(DateTimeFormat|RelativeTimeFormat)$/]"(
+        node,
+      ) {
+        context.report({ node, messageId: "date" });
+      },
+    };
+  },
+};
 // E28 S3b: one primitive per button family (docs/DESIGN.md). 268 hand-rolled buttons in 79 files had
 // drifted into as many looks, heights and hover styles; they were swept onto ui/Button, IconButton,
 // Pressable, Chip and DragHandle, and this keeps the next one out. The raw element lives only in the
@@ -351,6 +382,7 @@ const localRules = {
     "jsx-comment-text": jsxCommentText,
     "focus-safe-disabled": focusSafeDisabled,
     "button-primitive": buttonPrimitive,
+    "date-format": dateFormat,
     "input-primitive": inputPrimitive,
     "surface-role": surfaceRole,
     "literal-a11y-text": literalA11yText,
@@ -445,6 +477,7 @@ export default tseslint.config(
       "local/jsx-comment-text": "error",
       "local/focus-safe-disabled": "error",
       "local/button-primitive": "error",
+      "local/date-format": "error",
       "local/input-primitive": "error",
       "local/surface-role": "error",
       "local/literal-a11y-text": "error",