Player: seek and volume on /api/player

seek {ms} runs on the Spotify session task like a seek from the app (buffer
flushed, app notified). volume {value}|{delta} sets the player volume and
pushes it to the session, so the app's slider follows; setRemoteVolume now
also runs on the session task.

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

components/spotify/include/spotify.h

@@ -32,10 +32,13 @@ esp_err_t spotify_init(spotify_pcm_cb_t pcm, spotify_event_cb_t event, spotify_p
 // the client credentials are set; otherwise not advertised and a running session is ended.
 // A new device name ends the session and re-advertises.
 void spotify_apply(void);
-typedef enum { SPOTIFY_CMD_PLAY, SPOTIFY_CMD_PAUSE, SPOTIFY_CMD_NEXT, SPOTIFY_CMD_PREV } spotify_cmd_t;
+typedef enum {
+    SPOTIFY_CMD_PLAY, SPOTIFY_CMD_PAUSE, SPOTIFY_CMD_NEXT, SPOTIFY_CMD_PREV,
+    SPOTIFY_CMD_SEEK,       // arg: position in ms
+} spotify_cmd_t;
 // Control the running session (the Spotify app follows). Runs on the session task; waits until it
 // has run (max 1 s). Returns false without a session.
-bool spotify_command(spotify_cmd_t cmd);
+bool spotify_command(spotify_cmd_t cmd, uint32_t arg);
 // Suspend: end any running session (waits until its tasks are gone, max ~3 s) and refuse new ones.
 // Resume: back to what the config says. Used around firmware uploads.
 void spotify_suspend(bool suspend);