perf(ui-lvgl): lower clock when idle, fewer redraws, wake on BLE frames

- CPU at 80 MHz with the screen on after 2 s without touch or animation;
  a touch restores the full clock before its frame is drawn
- setText(): periodic refreshes no longer redraw unchanged labels
  (status bar, Home, lock, Diag, GPS, compass, clock, map, nav, nearby)
- map markers kept across the 3 s rebuild when unchanged
- app connected: the loop blocks until a BLE frame arrives
  (serialFrameArrived / hasPendingFrames) instead of polling every 2 ms
- PSRAM: boot frame sprite and the HTTP response buffer released

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/helpers/esp32/DualSerialInterface.h

@@ -52,6 +52,11 @@ public:
   bool isClientConnected() const override { return isBLEConnected(); }
 #endif
 
+  // USB can't tell without reading it.
+  bool hasPendingFrames() const override {
+    return (_ble_enabled && _ble.isConnected()) ? _ble.hasPendingFrames() : true;
+  }
+
   bool isWriteBusy() const override {
     return (_ble_enabled && _ble.isConnected()) ? _ble.isWriteBusy() : _usb.isWriteBusy();
   }