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/consts.dart

@@ -258,6 +258,33 @@ const int kMinTrackpadSpeed = 10;
 const int kDefaultTrackpadSpeed = 100;
 const int kMaxTrackpadSpeed = 1000;
 
+// relative mouse mode
+/// Throttle duration (in milliseconds) for updating pointer lock center during
+/// window move/resize events. Lower values provide more responsive updates but
+/// may cause performance issues during rapid window operations.
+const int kDefaultPointerLockCenterThrottleMs = 100;
+
+/// Minimum server version required for relative mouse mode (MOUSE_TYPE_MOVE_RELATIVE).
+/// Servers older than this version will ignore relative mouse events.
+///
+/// IMPORTANT: This value must be kept in sync with the Rust constant
+/// `MIN_VERSION_RELATIVE_MOUSE_MODE` in `src/common.rs`.
+const String kMinVersionForRelativeMouseMode = '1.4.5';
+
+/// Maximum delta value for relative mouse movement.
+/// Large values could cause issues with i32 overflow on server side,
+/// and no reasonable mouse movement should exceed this bound.
+///
+/// IMPORTANT: This value must be kept in sync with the Rust constant
+/// `MAX_RELATIVE_MOUSE_DELTA` in `src/server/input_service.rs`.
+const int kMaxRelativeMouseDelta = 10000;
+
+/// Debounce duration (in milliseconds) for relative mouse mode toggle.
+/// This prevents double-toggle from race condition between Rust rdev grab loop
+/// and Flutter keyboard handling. Value should be small enough to allow
+/// intentional quick toggles but large enough to prevent accidental double-triggers.
+const int kRelativeMouseModeToggleDebounceMs = 150;
+
 // incomming (should be incoming) is kept, because change it will break the previous setting.
 const String kKeyPrinterIncomingJobAction = 'printer-incomming-job-action';
 const String kValuePrinterIncomingJobDismiss = 'dismiss';