feat: now-playing liveness dot + admin-gated radio management (#1086)

* feat(now-playing): liveness indicator dot in the listening popover

Replace the raw "Nm ago" line in the "Who is listening?" popover with a
derived presence dot (green playing / amber paused / dim idle). The presence
is computed in one tested helper that unifies the playbackReport transport
state with the legacy getNowPlaying recency, instead of formatting a raw
timestamp inline. The dot carries the localized status as an aria-label and
tooltip so it is not conveyed by colour alone.

* feat(radio): gate station create/edit/delete behind Navidrome admin role

Navidrome >= 0.62 restricts internet-radio management to admins
(GHSA-jw24-qqrj-633c); non-admin requests fail. Hide Add Station, Search
Directory, the per-card edit chip and delete button for confirmed standard
Navidrome users via a canManageNavidromeRadio() helper on the existing
useNavidromeAdminRole framework. Admins, non-Navidrome servers and transient
states stay unrestricted; playback and favourites remain available to all.

* docs(changelog): now-playing status dot + admin-gated radio (#1086)

src/locales/en/nowPlaying.ts

@@ -3,7 +3,11 @@ export const nowPlaying = {
   title: 'Who is listening?',
   loading: 'Loading…',
   nobody: 'Nobody is currently listening.',
-  minutesAgo: '{{n}}m ago',
+  presence: {
+    playing: 'Playing',
+    paused: 'Paused',
+    idle: 'Idle',
+  },
   nothingPlaying: 'Nothing playing yet. Start a track!',
   aboutArtist: 'About the Artist',
   fromAlbum: 'From this Album',