Add VID/PID to build flags for ESP32 core + stronger visual indicators

- Set USB VID/PID via ESP32 core build flags (may work better than TinyUSB API)
- Heartbeat on LED 9 (amber pulse every 2s) confirms device is alive
- SysEx flashes ALL LEDs white for 200ms
- Regular MIDI flashes LED 0 white for 50ms

src/app_task.cpp

@@ -55,6 +55,7 @@ void AppTask::process_midi_event(const MidiEvent& event) {
         uint8_t cin = event.channel;
         uint8_t packet[3] = {event.data1, event.data2, 0};
         process_sysex_packet(packet, cin);
+        led_driver->flash_sysex();
         return;
     }