feat: Add relative mouse mode (#13928)

* feat: Add relative mouse mode

- Add "Relative Mouse Mode" toggle in desktop toolbar and bind to InputModel
- Implement relative mouse movement path: Flutter pointer deltas -> `type: move_relative` -> new `MOUSE_TYPE_MOVE_RELATIVE` in Rust
- In server input service, simulate relative movement via Enigo and keep latest cursor position in sync
- Track pointer-lock center in Flutter (local widget + screen coordinates) and re-center OS cursor after each relative move
- Update pointer-lock center on window move/resize/restore/maximize and when remote display geometry changes
- Hide local cursor when relative mouse mode is active (both Flutter cursor and OS cursor), restore on leave/disable
- On Windows, clip OS cursor to the window rect while in relative mode and release clip when leaving/turning off
- Implement platform helpers: `get_cursor_pos`, `set_cursor_pos`, `show_cursor`, `clip_cursor` (no-op clip/hide on Linux for now)
- Add keyboard shortcut Ctrl+Alt+Shift+M to toggle relative mode (enabled by default, works on all platforms)
- Remove `enable-relative-mouse-shortcut` config option - shortcut is now always available when keyboard permission is granted
- Handle window blur/focus/minimize events to properly release/restore cursor constraints
- Add MOUSE_TYPE_MASK constant and unit tests for mouse event constants

Note: Relative mouse mode state is NOT persisted to config (session-only).
Note: On Linux, show_cursor and clip_cursor are no-ops; cursor hiding is handled by Flutter side.

Signed-off-by: fufesou <linlong1266@gmail.com>

* feat(mouse): relative mouse mode, exit hint

Signed-off-by: fufesou <linlong1266@gmail.com>

* refact(relative mouse): shortcut

Signed-off-by: fufesou <linlong1266@gmail.com>

---------

Signed-off-by: fufesou <linlong1266@gmail.com>

flutter/lib/mobile/widgets/floating_mouse_widgets.dart

@@ -83,7 +83,10 @@ class _FloatingMouseWidgetsState extends State<FloatingMouseWidgets> {
           cursorModel: _cursorModel,
         ),
         if (virtualMouseMode.showVirtualJoystick)
-          VirtualJoystick(cursorModel: _cursorModel),
+          VirtualJoystick(
+            cursorModel: _cursorModel,
+            inputModel: _inputModel,
+          ),
         FloatingLeftRightButton(
           isLeft: true,
           inputModel: _inputModel,
@@ -674,12 +677,18 @@ class _QuarterCirclePainter extends CustomPainter {
   bool shouldRepaint(CustomPainter oldDelegate) => false;
 }
 
-// Virtual joystick sends the absolute movement for now.
-// Maybe we need to change it to relative movement in the future.
+// Virtual joystick can send either absolute movement (via updatePan)
+// or relative movement (via sendMobileRelativeMouseMove) depending on the
+// InputModel.relativeMouseMode setting.
 class VirtualJoystick extends StatefulWidget {
   final CursorModel cursorModel;
+  final InputModel inputModel;
 
-  const VirtualJoystick({super.key, required this.cursorModel});
+  const VirtualJoystick({
+    super.key,
+    required this.cursorModel,
+    required this.inputModel,
+  });
 
   @override
   State<VirtualJoystick> createState() => _VirtualJoystickState();
@@ -694,6 +703,10 @@ class _VirtualJoystickState extends State<VirtualJoystick> {
   final double _moveStep = 3.0;
   final double _speed = 1.0;
 
+  /// Scale factor for relative mouse movement sensitivity.
+  /// Higher values result in faster cursor movement on the remote machine.
+  static const double _kRelativeMouseScale = 3.0;
+
   // One-shot timer to detect a drag gesture
   Timer? _dragStartTimer;
   // Periodic timer for continuous movement
@@ -701,6 +714,9 @@ class _VirtualJoystickState extends State<VirtualJoystick> {
   Size? _lastScreenSize;
   bool _isPressed = false;
 
+  /// Check if relative mouse mode is enabled.
+  bool get _useRelativeMouse => widget.inputModel.relativeMouseMode.value;
+
   @override
   void initState() {
     super.initState();
@@ -746,6 +762,18 @@ class _VirtualJoystickState extends State<VirtualJoystick> {
     );
   }
 
+  /// Send movement delta to remote machine.
+  /// Uses relative mouse mode if enabled, otherwise uses absolute updatePan.
+  void _sendMovement(Offset delta) {
+    if (_useRelativeMouse) {
+      widget.inputModel.sendMobileRelativeMouseMove(
+          delta.dx * _kRelativeMouseScale, delta.dy * _kRelativeMouseScale);
+    } else {
+      // In absolute mode, use cursorModel.updatePan which tracks position.
+      widget.cursorModel.updatePan(delta, Offset.zero, false);
+    }
+  }
+
   void _stopSendEventTimer() {
     _dragStartTimer?.cancel();
     _continuousMoveTimer?.cancel();
@@ -773,7 +801,7 @@ class _VirtualJoystickState extends State<VirtualJoystick> {
           //    The movement is small for a gentle start.
           final initialDelta = _offsetToPanDelta(_offset);
           if (initialDelta.distance > 0) {
-            widget.cursorModel.updatePan(initialDelta, Offset.zero, false);
+            _sendMovement(initialDelta);
           }
 
           // 2. Start a one-shot timer to check if the user is holding for a drag.
@@ -784,10 +812,7 @@ class _VirtualJoystickState extends State<VirtualJoystick> {
             _continuousMoveTimer =
                 periodic_immediate(const Duration(milliseconds: 20), () async {
               if (_offset != Offset.zero) {
-                widget.cursorModel.updatePan(
-                    _offsetToPanDelta(_offset) * _moveStep * _speed,
-                    Offset.zero,
-                    false);
+                _sendMovement(_offsetToPanDelta(_offset) * _moveStep * _speed);
               }
             });
           });