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/pages/remote_page.dart

@@ -569,7 +569,9 @@ class _RemotePageState extends State<RemotePage> with WidgetsBindingObserver {
   }
 
   bool get showCursorPaint =>
-      !gFFI.ffiModel.isPeerAndroid && !gFFI.canvasModel.cursorEmbedded;
+      !gFFI.ffiModel.isPeerAndroid &&
+      !gFFI.canvasModel.cursorEmbedded &&
+      !gFFI.inputModel.relativeMouseMode.value;
 
   Widget getBodyForMobile() {
     final keyboardIsVisible = keyboardVisibilityController.isVisible;
@@ -808,6 +810,7 @@ class _RemotePageState extends State<RemotePage> with WidgetsBindingObserver {
                 bind.mainSetLocalOption(key: kOptionTouchMode, value: v);
               },
               virtualMouseMode: gFFI.ffiModel.virtualMouseMode,
+              inputModel: gFFI.inputModel,
             )));
   }