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/common/widgets/remote_input.dart

@@ -372,7 +372,10 @@ class _RawTouchGestureDetectorRegionState
         await ffi.cursorModel
             .move(_cacheLongPressPosition.dx, _cacheLongPressPosition.dy);
       }
-      await inputModel.sendMouse('down', MouseButtons.left);
+      // In relative mouse mode, skip mouse down - only send movement via sendMobileRelativeMouseMove
+      if (!inputModel.relativeMouseMode.value) {
+        await inputModel.sendMouse('down', MouseButtons.left);
+      }
       await ffi.cursorModel.move(d.localPosition.dx, d.localPosition.dy);
     } else {
       final offset = ffi.cursorModel.offset;
@@ -397,7 +400,12 @@ class _RawTouchGestureDetectorRegionState
     if (handleTouch && !_touchModePanStarted) {
       return;
     }
-    await ffi.cursorModel.updatePan(d.delta, d.localPosition, handleTouch);
+    // In relative mouse mode, send delta directly without position tracking.
+    if (inputModel.relativeMouseMode.value) {
+      await inputModel.sendMobileRelativeMouseMove(d.delta.dx, d.delta.dy);
+    } else {
+      await ffi.cursorModel.updatePan(d.delta, d.localPosition, handleTouch);
+    }
   }
 
   onOneFingerPanEnd(DragEndDetails d) async {
@@ -409,7 +417,10 @@ class _RawTouchGestureDetectorRegionState
       ffi.cursorModel.clearRemoteWindowCoords();
     }
     if (handleTouch) {
-      await inputModel.sendMouse('up', MouseButtons.left);
+      // In relative mouse mode, skip mouse up - matches the skipped mouse down in onOneFingerPanStart
+      if (!inputModel.relativeMouseMode.value) {
+        await inputModel.sendMouse('up', MouseButtons.left);
+      }
     }
   }