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);