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.dart

@@ -1011,13 +1011,15 @@ makeMobileActionsOverlayEntry(VoidCallback? onHide, {FFI? ffi}) {
   });
 }
 
-void showToast(String text, {Duration timeout = const Duration(seconds: 3)}) {
+void showToast(String text,
+    {Duration timeout = const Duration(seconds: 3),
+    Alignment alignment = const Alignment(0.0, 0.8)}) {
   final overlayState = globalKey.currentState?.overlay;
   if (overlayState == null) return;
   final entry = OverlayEntry(builder: (context) {
     return IgnorePointer(
         child: Align(
-            alignment: const Alignment(0.0, 0.8),
+            alignment: alignment,
             child: Container(
               decoration: BoxDecoration(
                 color: MyTheme.color(context).toastBg,
@@ -4069,3 +4071,23 @@ String decode_http_response(http.Response resp) {
 bool peerTabShowNote(PeerTabIndex peerTabIndex) {
   return peerTabIndex == PeerTabIndex.ab || peerTabIndex == PeerTabIndex.group;
 }
+
+// TODO: We should support individual bits combinations in the future.
+// But for now, just keep it simple, because the old code only supports single button.
+// No users have requested multi-button support yet.
+String mouseButtonsToPeer(int buttons) {
+  switch (buttons) {
+    case kPrimaryMouseButton:
+      return 'left';
+    case kSecondaryMouseButton:
+      return 'right';
+    case kMiddleMouseButton:
+      return 'wheel';
+    case kBackMouseButton:
+      return 'back';
+    case kForwardMouseButton:
+      return 'forward';
+    default:
+      return '';
+  }
+}