Enhance Bluetooth functionality and GUI integration

- Add Bluetooth device list management with discovery and pairing support
- Implement volume control with AVRC integration
- Add system event handling for GUI-to-Bluetooth communication
- Enhance menu system with device selection and volume control pages
- Improve memory management by making menu creation lazy
- Add proper event subscription between GUI and Bluetooth modules
- Update filter coefficient for improved audio clicking behavior

🤖 Generated with [Claude Code](https://claude.ai/code)

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

main/bt_app.h

@@ -10,6 +10,7 @@
 #include <stdint.h>
 #include <stdbool.h>
 #include <stdio.h>
+#include "esp_bt_defs.h"
 
 /* log tag */
 #define BT_APP_CORE_TAG             "BT_APP_CORE"
@@ -67,4 +68,41 @@ void bt_app_task_shut_down(void);
 
 void bt_app_init(void);
 
+/* Bluetooth device management for GUI */
+#define MAX_BT_DEVICES 20
+#define MAX_BT_NAME_LEN 32
+
+typedef struct {
+    esp_bd_addr_t bda;
+    char name[MAX_BT_NAME_LEN];
+    bool is_paired;
+    int rssi;
+} bt_device_info_t;
+
+typedef struct {
+    bt_device_info_t devices[MAX_BT_DEVICES];
+    int count;1 
+    bool discovery_active;
+} bt_device_list_t;
+
+/* Get current device list for GUI display */
+bt_device_list_t* bt_get_device_list(void);
+
+/* Start device discovery */
+bool bt_start_discovery(void);
+
+/* Stop device discovery */
+bool bt_stop_discovery(void);
+
+/* Connect to specific device by index in device list */
+bool bt_connect_device(int device_index);
+
+/* Clear discovered devices (keep paired devices) */
+void bt_clear_discovered_devices(void);
+
+/* Volume control functions */
+void bt_volume_up(void);
+void bt_volume_down(void);
+int bt_get_current_volume(void);
+
 #endif /* __BT_APP_CORE_H__ */