Add visual MIDI activity indicator

Flash LED 0 white for 30ms on any MIDI input.
Visible without serial - helps diagnose whether MIDI
is arriving when connected to iPad with Loopy Pro.

src/app_task.cpp

@@ -45,6 +45,10 @@ void AppTask::update() {
 void AppTask::process_midi_event(const MidiEvent& event) {
     Serial.printf("[APP] MIDI IN: Type=%d Ch=%d Data1=%d Data2=%d\n",
                   event.type, event.channel, event.data1, event.data2);
+
+    // Flash LED 0 white briefly on ANY MIDI input - visual activity indicator
+    // (visible without serial when connected to iPad)
+    led_driver->flash_activity();
     
     uint8_t led_index = 0xFF;
     uint8_t midi_channel = event.channel;