Convert to PlatformIO/Arduino format

- Replace ESP-IDF structure with PlatformIO (platformio.ini)
- Move source files to src/ and include/
- Move libraries to lib/ directory
- Replace FreeRTOS with Arduino task/vTaskDelay
- Use Arduino MIDI library instead of raw TinyUSB
- Dual-core: MIDI on core 0, controller on core 1

include/app_task.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include <cstdint>
+#include "midi_transport.h"
+#include "led_stub.h"
+#include "switch_stub.h"
+
+struct PadMapping {
+    uint8_t physical_switch;
+    uint8_t midi_channel;
+    uint8_t midi_note;
+    uint8_t led_index;
+};
+
+class AppTask {
+public:
+    AppTask(LedStub* led, SwitchStub* sw, UsbMidiTransport* midi);
+    
+    void begin();
+    void update();
+    
+private:
+    LedStub* led_driver;
+    SwitchStub* switch_driver;
+    UsbMidiTransport* midi_transport;
+    
+    static const uint8_t NUM_PADS = 10;
+    PadMapping pad_mapping[NUM_PADS];
+    bool last_switch_state[NUM_PADS];
+    
+    void process_midi_event(const MidiEvent& event);
+    void process_switch_event(uint8_t switch_id, bool pressed);
+};