feat(navidrome): Instant Mix probe, ping identity, and AudioMuse UX

Parse type and serverVersion from Subsonic ping; persist per-server identity and
run a background Instant Mix probe (random songs + getSimilarSongs) for
Navidrome 0.60+. Hide the AudioMuse server toggle when the probe returns no
similar tracks; clear prefs when the server is no longer eligible.

Artist pages fall back to Last.fm similar artists when AudioMuse is enabled but
the server returns none. Instant Mix failures set a per-server issue flag with a
settings warning and toast. Settings description links the official plugin repo
via i18n Trans.

src/locales/en.ts

@@ -96,6 +96,7 @@ export const enTranslation = {
     enqueueAlbum: 'Enqueue Album',
     startRadio: 'Start Radio',
     instantMix: 'Instant Mix',
+    instantMixFailed: 'Could not build Instant Mix — server or plugin error.',
     lfmLove: 'Love on Last.fm',
     lfmUnlove: 'Unlove on Last.fm',
     favorite: 'Favorite',
@@ -400,7 +401,9 @@ export const enTranslation = {
     serverCompatible: 'Compatible with: Navidrome · Gonic · Airsonic · Subsonic',
     audiomuseTitle: 'AudioMuse-AI (Navidrome)',
     audiomuseDesc:
-      'Turn on if this server has the AudioMuse-AI Navidrome plugin configured. Enables Instant Mix from tracks and uses server-side similar artists instead of Last.fm on artist pages.',
+      'Turn on if this server has the <pluginLink>AudioMuse-AI Navidrome plugin</pluginLink> configured. Enables Instant Mix from tracks and uses server-side similar artists instead of Last.fm on artist pages.',
+    audiomuseIssueHint:
+      'Instant Mix failed recently — check the Navidrome plugin and AudioMuse API. Similar artists fall back to Last.fm when the server returns none.',
     connected: 'Connected',
     failed: 'Failed',
     eqTitle: 'Equalizer',