Player: auto mode fails over between Spotify and HLS

No Spotify session (or paused, with failover_on_pause) for failover_delay_s
switches to HLS; Spotify playing switches back within ~1 s. Switches fade
out/in over 30 ms and flush the ring. HLS is suspended while Spotify plays.
cspot is held back instead of drained while it isn't Spotify's turn: it keeps
decoding on pause, so dropping its frames let it race through the queue.

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

components/spotify/spotify.cpp

@@ -129,6 +129,11 @@ extern "C" void spotify_pause(bool pause)
     }
 }
 
+extern "C" bool spotify_session_active(void)
+{
+    return s_handler != nullptr;
+}
+
 const char *spotify_state(void)
 {
     return s_state.load();