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/system.c

@@ -166,4 +166,36 @@ void system_notifyAll(uint32_t eventBits) {
         }
         xSemaphoreGive(em->mutex);
     }
+}
+
+void system_requestBtRefresh(void) {
+    ESP_LOGI("system", "BT Refresh requested");
+    system_notifyAll(EM_EVENT_BT_REFRESH);
+}
+
+void system_requestBtConnect(int device_index) {
+    xSemaphoreTake(_eventManager.mutex, portMAX_DELAY);
+    _systemState.btDeviceIndex = device_index;
+    xSemaphoreGive(_eventManager.mutex);
+    
+    ESP_LOGI("system", "BT Connect requested for device %d", device_index);
+    system_notifyAll(EM_EVENT_BT_CONNECT);
+}
+
+int system_getBtDeviceIndex(void) {
+    int index;
+    xSemaphoreTake(_eventManager.mutex, portMAX_DELAY);
+    index = _systemState.btDeviceIndex;
+    xSemaphoreGive(_eventManager.mutex);
+    return index;
+}
+
+void system_requestVolumeUp(void) {
+    ESP_LOGI("system", "Volume Up requested");
+    system_notifyAll(EM_EVENT_VOLUME_UP);
+}
+
+void system_requestVolumeDown(void) {
+    ESP_LOGI("system", "Volume Down requested");
+    system_notifyAll(EM_EVENT_VOLUME_DOWN);
 }