Player: GET /api/player with Spotify now-playing info

Source, state, artist/title/album, position and duration (from cspot's
playback state, so it matches the app), volume and `can`. cspot patch 0004
adds read getters for the playback state and context.

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

components/spotify/include/spotify.h

@@ -42,6 +42,14 @@ void spotify_suspend(bool suspend);
 void spotify_set_volume(int pct);
 // A session is running (an app is connected to this device).
 bool spotify_session_active(void);
+
+typedef struct {
+    char artist[96], title[128], album[96];     // "" until the first track is audible
+    uint32_t duration_ms, position_ms;          // position as the Spotify app sees it
+    bool paused;
+} spotify_track_t;
+// The current track of the running session. False without a session.
+bool spotify_now_playing(spotify_track_t *t);
 // Current state for status.spotify_state.
 const char *spotify_state(void);