feat(ui): detect USB-CDC connection for Auto mute (BLE or USB)

USB client presence IS detectable on nRF52 after all: (bool)Serial ==
tud_cdc_n_connected() (DTR — a host has the CDC port open). Add
isClientConnected() = BLE-bonded OR USB-CDC-open; DualSerialInterface
overrides it, base defaults to isConnected() (single-transport unchanged).

Wire it so each consumer gets the right signal:
- hasConnection() ← isClientConnected(): Auto buzzer mute + message-wake now
  trigger on BLE or an open USB port (PR #14's intent, done correctly), but
  not on charging-only (no host → DTR low).
- BT status indicator + pairing PIN stay on isBLEConnected() (BLE-specific).

Caveat: a plain serial monitor also asserts DTR, so it counts as connected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/companion_radio/MyMesh.cpp

@@ -2497,12 +2497,11 @@ void MyMesh::loop() {
   }
 
 #ifdef DISPLAY_CLASS
-  // Use BLE-bonded state, not isConnected(): on a dual (BLE+USB) interface
-  // isConnected() is hardcoded true (USB is always a send fallback), and USB
-  // client presence isn't detectable anyway. hasConnection() drives the BT
-  // status indicator, message-wake and the Auto buzzer mute — all of which
-  // mean "a companion app is connected", i.e. BLE bonded.
-  if (_ui) _ui->setHasConnection(_serial->isBLEConnected());
+  // "a companion app is connected over any transport" — BLE bonded or a host
+  // holding the USB-CDC port open. Drives Auto buzzer mute + message-wake.
+  // (Not isConnected(): a dual interface hardcodes that true as a send
+  // fallback. The BT status indicator and pairing PIN use isBLEConnected().)
+  if (_ui) _ui->setHasConnection(_serial->isClientConnected());
 #endif
 }
 

examples/companion_radio/ui-new/UITask.cpp

@@ -456,7 +456,7 @@ class HomeScreen : public UIScreen {
     int leftmostX = battLeftX;
     if (_task->isSerialEnabled()) {
       int btX = battLeftX - ind - ind_gap;
-      if (_task->hasConnection()) {
+      if (_task->isBLEConnected()) {   // BT icon reflects BLE link, not USB
         display.setColor(DisplayDriver::LIGHT);
         display.fillRect(btX, 0, ind, ind_h);
         display.setColor(DisplayDriver::DARK);

src/helpers/BaseSerialInterface.h

@@ -18,6 +18,12 @@ public:
   // 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(); }
+  // True when a companion app is actually connected over *any* transport
+  // (BLE bonded or a host holding the USB-CDC port open). Distinct from
+  // isConnected(), which a dual interface hardcodes true as a send fallback.
+  // Used for app-connected UI (Auto buzzer mute, message-wake). Default =
+  // isConnected() so single-transport interfaces are unchanged.
+  virtual bool isClientConnected() 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

@@ -33,6 +33,10 @@ public:
   bool isConnected() const override { return true; }
   // True only when a BLE companion app is paired and connected.
   bool isBLEConnected() const override { return _ble_enabled && _ble.isConnected(); }
+  // A companion app is actually connected if BLE is bonded OR a host holds the
+  // USB-CDC port open. (bool)Serial == tud_cdc_n_connected() (DTR asserted) — a
+  // serial monitor counts too, but charging-only (no host) does not.
+  bool isClientConnected() const override { return isBLEConnected() || (bool)Serial; }
 
   bool isWriteBusy() const override {
     return (_ble_enabled && _ble.isConnected()) ? _ble.isWriteBusy() : _usb.isWriteBusy();