fix: correct dual serial interface — routing, UI state, debug logging

- isConnected() returns true always (USB always available, mesh can send)
- isBLEConnected() added to BaseSerialInterface for BLE-specific UI/buzzer state
- MyMesh uses isBLEConnected() to update UITask connection indicator
- Disable BLE_DEBUG_LOGGING in dual build (would corrupt USB stream)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/helpers/BaseSerialInterface.h

@@ -14,6 +14,10 @@ public:
   virtual bool isEnabled() const = 0;
 
   virtual bool isConnected() const = 0;
+  // Returns true when a BLE companion app is connected. Default delegates to
+  // isConnected(); override in dual-interface wrappers that always report
+  // isConnected()=true but still need to distinguish BLE from USB state.
+  virtual bool isBLEConnected() const { return isConnected(); }
 
   virtual bool isWriteBusy() const = 0;
   virtual size_t writeFrame(const uint8_t src[], size_t len) = 0;

src/helpers/nrf52/DualSerialInterface.h

@@ -22,8 +22,10 @@ public:
   void disable() override { _ble.disable(); _usb.disable(); }
   bool isEnabled() const override { return _ble.isEnabled() || _usb.isEnabled(); }
 
-  // Reports BLE connection state (used for UI indicator and buzzer Auto mode)
-  bool isConnected() const override { return _ble.isConnected(); }
+  // Always true — USB is always available as fallback, so the mesh can send.
+  bool isConnected() const override { return true; }
+  // True only when a BLE companion app is paired and connected.
+  bool isBLEConnected() const override { return _ble.isConnected(); }
 
   bool isWriteBusy() const override {
     return _ble.isConnected() ? _ble.isWriteBusy() : _usb.isWriteBusy();