Implement full NFC-URL streaming architecture

- rc522: add NDEF URL reading via WUPA+cascade anticoll+SELECT+TLV parse
- AudioPlayer: route http:// URIs to esp_player_http_music_play()
- CacheManager: download URLs async to /sdcard/cache/{hash}.{ext}
- TagPlayerControl: NDEF URL → cache-or-stream; fallback to UID local mapping
- RFIDInterface: wire ndef_callback, expose combined uid+url callback
- WebInterface: add /api/info, /api/storage, /api/playback, /api/bluetooth
- BluetoothInterface: stub for A2DP speaker mode
- Networking: WiFi connect with 30s timeout instead of blocking forever
- main.cpp: re-enable WiFi + web server, fix init order, add CacheManager
- www/: full SPA — playback controls, device info, storage, BT pairing
- .gitmodules: switch rc522 submodule URL to HTTPS

https://claude.ai/code/session_01MhAnh68DPNnTjcKqjNYDmk

device/main/BluetoothInterface.hpp

@@ -0,0 +1,19 @@
+#pragma once
+
+/* Stub for Bluetooth A2DP speaker mode.
+ * When enabled the ESP32 advertises itself as a BT audio sink so a phone
+ * can pair and route music through the Kidsbox speaker.
+ * Full implementation requires the esp_bt + esp_a2dp components to be
+ * added to the build and CONFIG_BT_ENABLED=y in sdkconfig. */
+class BluetoothInterface {
+public:
+    BluetoothInterface();
+
+    bool isEnabled() const { return enabled; }
+
+    /* Toggle BT speaker mode on/off.  Returns whether BT is now enabled. */
+    bool setEnabled(bool on);
+
+private:
+    bool enabled = false;
+};