feat(discovery): Navidrome AudioMuse-AI client integration and library scoping

Add per-server toggle for AudioMuse-style discovery: Instant Mix via
getSimilarSongs, server similar artists instead of Last.fm on artist pages,
and higher similar-artist count in Now Playing when enabled.

When browsing a single music folder, filter similar/top song results by
album ids from that scope (Navidrome does not apply musicFolderId to those
endpoints). Request larger similar batches under a narrow scope.

Keep radio-from-track as two blocks (shuffled top songs, then shuffled
similar-by-artist) so it differs from Instant Mix. Show Alpha badge beside
the setting. Add research/ to .gitignore.

src/locales/en.ts

@@ -95,6 +95,7 @@ export const enTranslation = {
     addToQueue: 'Add to Queue',
     enqueueAlbum: 'Enqueue Album',
     startRadio: 'Start Radio',
+    instantMix: 'Instant Mix',
     lfmLove: 'Love on Last.fm',
     lfmUnlove: 'Unlove on Last.fm',
     favorite: 'Favorite',
@@ -397,6 +398,9 @@ export const enTranslation = {
     testBtn: 'Test Connection',
     testingBtn: 'Testing…',
     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.',
     connected: 'Connected',
     failed: 'Failed',
     eqTitle: 'Equalizer',