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>

src/server/connection.rs

@@ -5173,9 +5173,13 @@ impl Retina {
 
     #[inline]
     fn on_mouse_event(&mut self, e: &mut MouseEvent, current: usize) {
-        let evt_type = e.mask & 0x7;
-        if evt_type == crate::input::MOUSE_TYPE_WHEEL {
-            // x and y are always 0, +1 or -1
+        let evt_type = e.mask & crate::input::MOUSE_TYPE_MASK;
+        // Delta-based events do not contain absolute coordinates.
+        // Avoid applying Retina coordinate scaling to them.
+        if evt_type == crate::input::MOUSE_TYPE_WHEEL
+            || evt_type == crate::input::MOUSE_TYPE_TRACKPAD
+            || evt_type == crate::input::MOUSE_TYPE_MOVE_RELATIVE
+        {
             return;
         }
         let Some(d) = self.displays.get(current) else {
@@ -5421,6 +5425,9 @@ mod raii {
                     .unwrap()
                     .on_connection_close(self.0);
             }
+            // Clear per-connection state to avoid stale behavior if conn ids are reused.
+            #[cfg(not(any(target_os = "android", target_os = "ios")))]
+            clear_relative_mouse_active(self.0);
             AUTHED_CONNS.lock().unwrap().retain(|c| c.conn_id != self.0);
             let remote_count = AUTHED_CONNS
                 .lock()