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

include/led_stub.h

@@ -35,6 +35,8 @@ private:
     bool initialized;
     uint32_t activity_off_time = 0;
     uint8_t saved_r = 0, saved_g = 0, saved_b = 0;
+    uint32_t heartbeat_time = 0;
+    uint8_t heartbeat_phase = 0;
     
 public:
     DefaultLedStub();