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/platformio.ini

@@ -7,6 +7,7 @@ monitor_speed = 115200
 
 build_flags =
     -DARDUINO_USB_CDC_ON_BOOT=1
+    -UARDUINO_USB_MODE -DARDUINO_USB_MODE=0
     -DBOARD_HAS_PSRAM
     ; LVGL config via build flags
     -DLV_CONF_SKIP