fix(ui): hasConnection() should track BLE bond, not isConnected()

PR #14 (advert sound) changed setHasConnection() from isBLEConnected() to
isConnected() to also mute on USB — but DualSerialInterface::isConnected() is
hardcoded true (USB is always a send fallback) and ArduinoSerialInterface's is
too ("no way of knowing"), so USB-client presence is undetectable. The result
on every dual build: hasConnection() always true, which broke everything that
means "a companion app is connected":
- Auto buzzer mute → permanently muted (Auto behaved like Off)
- BT status indicator → always shown connected
- message-wake → display never woke for new messages

Revert to isBLEConnected() (the only detectable client signal). Mute-on-USB
isn't achievable; Buzzer = Off covers that case manually.

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

examples/companion_radio/MyMesh.cpp

@@ -2497,7 +2497,12 @@ void MyMesh::loop() {
   }
 
 #ifdef DISPLAY_CLASS
-  if (_ui) _ui->setHasConnection(_serial->isConnected());
+  // 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());
 #endif
 }