Add 3-finger swipe gesture for Alt-Tab functionality

[Copilot]

Implements 3-finger swipe up/down on mobile mousepad to trigger the "show all apps" view on desktop.

Protocol Extension

• Added ThreeFingerSwipe action (code 10) with SwipeDirection enum (Up=0, Down=1)
• Binary format: [10, direction]

Backend (Rust)

actions.rs:

• New action type with decoding logic and unit tests
• Safe fallback to Up for invalid direction values

mobile_controller.rs:

• Handler triggers existing AltTab key combo (Super on Linux, Win+Tab on Windows, Control+Up on macOS)
• Both directions show the same apps overview

Frontend (Dart)

input.dart:

• Encoding functions: threeFingerSwipeUp() / threeFingerSwipeDown()

mousepad.dart:

• 3-finger detection via GestureDetector.onScaleStart (pointerCount == 3)
• Velocity-based direction detection in onScaleEnd (threshold: 100 px/s vertical)
• Cancels drag mode timer when 3 fingers detected

void _handleScaleEnd(ScaleEndDetails details) {
  if (isThreeFingerSwipe) {
    final dy = details.velocity.pixelsPerSecond.dy;
    if (dy.abs() > 100) {
      ServerConnector.sendInput(
        dy < 0 ? Input.threeFingerSwipeUp() : Input.threeFingerSwipeDown()
      );
    }
  }
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Mousepad swipes</issue_title>
<issue_description>Add mousepad swipe functionality:

• 3 finger swipe up/down - same as the alt tab shortcut - should show all opened apps
• 2 finger swipe left: go back
• 2 finger swipe right: go right
• pinch: zoom or fullscreen
• 2 finger touch - same as right mouse button</issue_description>

<agent_instructions>Implement only the 3 finger swipe up / down feature</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Mousepad swipes #38

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