fix(bluetooth): wait for SCO link before Oboe restart + detect A2DP devices

Three fixes for Bluetooth audio not working:

1. is_bluetooth_available() now checks for TYPE_BLUETOOTH_A2DP (8) in
   addition to TYPE_BLUETOOTH_SCO (7) — many headsets only register as
   A2DP until SCO is explicitly started.

2. set_bluetooth_sco(on=true) polls isBluetoothScoOn() for up to 3s
   before restarting Oboe. startBluetoothSco() is async — the SCO link
   takes 500ms-2s to establish. Without waiting, Oboe opens against
   earpiece and audio goes nowhere.

3. Frontend skips redundant set_speakerphone(false) when transitioning
   to BT — start_bluetooth_sco() handles speaker-off internally,
   avoiding a double Oboe restart.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/src/main.ts

@@ -939,15 +939,17 @@ async function cycleAudioRoute() {
     const idx = routes.indexOf(currentAudioRoute);
     const next = routes[(idx + 1) % routes.length];
 
-    // Tear down current route
+    // Tear down current route, then activate next.
+    // start_bluetooth_sco() already calls setSpeakerphoneOn(false)
+    // internally, so we skip the separate speakerphone toggle when
+    // transitioning to BT to avoid a redundant Oboe restart.
     if (currentAudioRoute === "bluetooth") {
       await invoke("set_bluetooth_sco", { on: false });
     }
-    // Activate next route
     if (next === "speaker") {
       await invoke("set_speakerphone", { on: true });
     } else if (next === "bluetooth") {
-      await invoke("set_speakerphone", { on: false });
+      // BT start handles speaker-off internally + waits for SCO link
       await invoke("set_bluetooth_sco", { on: true });
     } else {
       // earpiece — turn everything off