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/lang/en.rs

@@ -262,5 +262,10 @@ pub static ref T: std::collections::HashMap<&'static str, &'static str> =
         ("disable-udp-tip", "Controls whether to use TCP only.\nWhen this option enabled, RustDesk will not use UDP 21116 any more, TCP 21116 will be used instead."),
         ("server-oss-not-support-tip", "NOTE: RustDesk server OSS doesn't include this feature."),
         ("note-at-conn-end-tip", "Ask for note at end of connection"),
+        ("rel-mouse-not-supported-peer-tip", "Relative Mouse Mode is not supported by the connected peer."),
+        ("rel-mouse-not-ready-tip", "Relative Mouse Mode is not ready yet. Please try again."),
+        ("rel-mouse-lock-failed-tip", "Failed to lock cursor. Relative Mouse Mode has been disabled."),
+        ("rel-mouse-exit-{}-tip", "Press {} to exit."),
+        ("rel-mouse-permission-lost-tip", "Keyboard permission was revoked. Relative Mouse Mode has been disabled."),
     ].iter().cloned().collect();
 }