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/desktop/pages/remote_tab_page.dart

@@ -135,7 +135,13 @@ class _ConnectionTabPageState extends State<ConnectionTabPage> {
       body: DesktopTab(
         controller: tabController,
         onWindowCloseButton: handleWindowCloseButton,
-        tail: const AddButton(),
+        tail: Row(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            _RelativeMouseModeHint(tabController: tabController),
+            const AddButton(),
+          ],
+        ),
         selectedBorderColor: MyTheme.accent,
         pageViewBuilder: (pageView) => pageView,
         labelGetter: DesktopTab.tablabelGetter,
@@ -374,6 +380,8 @@ class _ConnectionTabPageState extends State<ConnectionTabPage> {
       loopCloseWindow();
     }
     ConnectionTypeState.delete(id);
+    // Clean up relative mouse mode state for this peer.
+    stateGlobal.relativeMouseModeState.remove(id);
     _update_remote_count();
   }
 
@@ -548,3 +556,69 @@ class _ConnectionTabPageState extends State<ConnectionTabPage> {
     return returnValue;
   }
 }
+
+/// A widget that displays a hint in the tab bar when relative mouse mode is active.
+/// This helps users remember how to exit relative mouse mode.
+class _RelativeMouseModeHint extends StatelessWidget {
+  final DesktopTabController tabController;
+
+  const _RelativeMouseModeHint({Key? key, required this.tabController})
+      : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Obx(() {
+      // Check if there are any tabs
+      if (tabController.state.value.tabs.isEmpty) {
+        return const SizedBox.shrink();
+      }
+
+      // Get current selected tab's RemotePage
+      final selectedTabInfo = tabController.state.value.selectedTabInfo;
+      if (selectedTabInfo.page is! RemotePage) {
+        return const SizedBox.shrink();
+      }
+
+      final remotePage = selectedTabInfo.page as RemotePage;
+      final String peerId = remotePage.id;
+
+      // Use global state to check relative mouse mode (synced from InputModel).
+      // This avoids timing issues with FFI registration.
+      final isRelativeMouseMode =
+          stateGlobal.relativeMouseModeState[peerId] ?? false;
+
+      if (!isRelativeMouseMode) {
+        return const SizedBox.shrink();
+      }
+
+      return Container(
+        padding: const EdgeInsets.symmetric(horizontal: 8, vertical: 4),
+        margin: const EdgeInsets.only(right: 8),
+        decoration: BoxDecoration(
+          color: Colors.orange.withOpacity(0.2),
+          borderRadius: BorderRadius.circular(4),
+          border: Border.all(color: Colors.orange.withOpacity(0.5)),
+        ),
+        child: Row(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Icon(
+              Icons.mouse,
+              size: 14,
+              color: Colors.orange[700],
+            ),
+            const SizedBox(width: 4),
+            Text(
+              translate(
+                  'rel-mouse-exit-{${isMacOS ? "Cmd+G" : "Ctrl+Alt"}}-tip'),
+              style: TextStyle(
+                fontSize: 11,
+                color: Colors.orange[700],
+              ),
+            ),
+          ],
+        ),
+      );
+    });
+  }
+}