fix: bluetooth no results, audio waveform leak, and mode switch cleanup

- Change 'already_running' to 'already_scanning' status in bluetooth_v2
  so frontend recognizes the response and connects the SSE stream
- Hide pagerScopePanel and sensorScopePanel in switchMode() to prevent
  audio waveform bars leaking into other modes
- Clear devices Map, pendingDeviceIds Set, and UI in BluetoothMode.destroy()
  to prevent memory accumulation on repeated mode switches

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

static/js/modes/bluetooth.js

@@ -1784,6 +1784,13 @@ const BluetoothMode = (function() {
      */
     function destroy() {
         stopEventStream();
+        devices.clear();
+        pendingDeviceIds.clear();
+        if (deviceContainer) {
+            deviceContainer.innerHTML = '';
+        }
+        const countEl = document.getElementById('btDeviceListCount');
+        if (countEl) countEl.textContent = '0';
     }
 })();