Step 7.5b2: Spotify Connect zeroconf and login work (PCM counted only)

- Zeroconf on our httpd (GET/POST /spotify_info, form body URL-decoded)
  and _spotify-connect._tcp via our mDNS (VERSION, CPath, Stack TXT).
- Session task: waits for the app's login blob, connects with the
  user's client ID/secret and the configured bitrate (96/160/320 ->
  OGG_VORBIS_*), SpircHandler, events logged; cspot exceptions caught.
  Starts when source.mode is spotify/auto and credentials are set.
- bell::bellGlobalLogger was NULL: every CSPOT_LOG crashed the board
  right after addUser (Load access fault in Session.cpp:67 /
  LoginBlob.cpp:58). cspot/bell logs now go to esp_log (UART + syslog);
  signed CDN URL tokens are cut from the log.
- esp_audio_codec's own Vorbis decoder clashed with bell's Tremor
  symbols: CONFIG_AUDIO_DECODER_VORBIS_SUPPORT / SIMPLE_DEC_OGG off.
- status.spotify_state from the session (waiting / connecting /
  connected / login failed / error / disabled / no client credentials).
- Verified from a Mac: device appears, "connected as <user>", access
  token fetched with the user's client credentials, track info, audio
  key, CDN URL (the step broken upstream), PCM decoded. Without
  backpressure a track decodes in ~26 s (b3 feeds the ring).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

components/spotify/include/spotify.h

@@ -7,7 +7,11 @@
 extern "C" {
 #endif
 
-esp_err_t spotify_init(const char *device_name);
+// Registers the zeroconf endpoints (/spotify_info) and, when source.mode is spotify/auto and the
+// client credentials are set, advertises the device and starts the session task.
+esp_err_t spotify_init(void);
+// Current state for status.spotify_state.
+const char *spotify_state(void);
 
 #ifdef __cplusplus
 }