Player: runtime source override and play-a-URL on /api/player

The player now owns the effective mode (config or override) and passes it
down: spotify_apply(enable) and hls_set_url(url) instead of both reading
source.mode from the config. /api/player/source forces spotify|hls|off
(config returns to the saved mode), /api/player/url plays an m3u8 now;
neither is saved. GET reports it in `forced`.

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

main/hls.h

@@ -10,7 +10,11 @@
 // Receives segment data in chunks (MPEG-TS etc.). Return false to abort the segment.
 typedef bool (*hls_sink_t)(const uint8_t *data, size_t len, bool segment_start);
 
+#define HLS_URL_MAX 384
+
 esp_err_t hls_start(hls_sink_t sink);
+// The playlist URL to play ("" = none). A different URL restarts fetching with it.
+void hls_set_url(const char *url);
 // Stop fetching (current download is abandoned) until resumed. Reasons are independent:
 // fetching runs only when no reason holds it.
 #define HLS_SUSPEND_OTA      (1 << 0)