Add USB HID keyboard input via touch gestures

The SC01 Plus now acts as a composite USB device (CDC serial + HID keyboard).
Touch gestures on the screen send keystrokes to the computer:

- Swipe up/down: arrow keys
- Swipe right: Enter
- Swipe left: Shift+Tab (change mode in Claude Code)
- Double-tap logo: Ctrl+Space (voice mode)
- Tap and hold: Space (held while touching)

Gesture-to-key mapping is defined as a simple config table in hid.cpp.
Touch detection uses a state machine with configurable thresholds in touch.cpp.

Switches to TinyUSB composite mode (ARDUINO_USB_MODE=0). Flashing now
requires flash.sh which handles the DTR/RTS bootloader reset.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

firmware/src/hid.h

@@ -0,0 +1,17 @@
+#pragma once
+#include <Arduino.h>
+
+// Gesture types
+enum gesture_t {
+    GESTURE_NONE,
+    GESTURE_SWIPE_UP,
+    GESTURE_SWIPE_DOWN,
+    GESTURE_SWIPE_LEFT,
+    GESTURE_SWIPE_RIGHT,
+    GESTURE_DOUBLE_TAP_LOGO,
+    GESTURE_HOLD_START,
+    GESTURE_HOLD_END,
+};
+
+void hid_init(void);
+void hid_on_gesture(gesture_t gesture);