fix: remove dead Spotify credentials; backend URL falls back to NS_BACKEND_URL env on login and auto-login

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/state.js

@@ -1,6 +1,6 @@
 // Shared mutable state — imported as { S } in every module
 export const S = {
-  cfg: { url: '', user: '', pass: '', lastfmKey: '', backendUrl: '', spotifyClientId: '', spotifyClientSecret: '' },
+  cfg: { url: '', user: '', pass: '', lastfmKey: '', backendUrl: '' },
 
   // Playback
   queue: [],