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/utils/relative_mouse_accumulator.dart

@@ -0,0 +1,58 @@
+/// A small helper for accumulating fractional mouse deltas and emitting integer deltas.
+///
+/// Relative mouse mode uses integer deltas on the wire, but Flutter pointer deltas
+/// are doubles. This accumulator preserves sub-pixel movement by carrying the
+/// fractional remainder across events.
+class RelativeMouseDelta {
+  final int x;
+  final int y;
+
+  const RelativeMouseDelta(this.x, this.y);
+}
+
+/// Accumulates fractional mouse deltas and returns integer deltas when available.
+class RelativeMouseAccumulator {
+  double _fracX = 0.0;
+  double _fracY = 0.0;
+
+  /// Adds a delta and returns an integer delta when at least one axis reaches a
+  /// magnitude of 1px (after truncation towards zero).
+  ///
+  /// If [maxDelta] is > 0, the returned integer delta is clamped to
+  /// [-maxDelta, maxDelta] on each axis.
+  RelativeMouseDelta? add(
+    double dx,
+    double dy, {
+    required int maxDelta,
+  }) {
+    // Guard against misuse: negative maxDelta would silently disable clamping.
+    assert(maxDelta >= 0, 'maxDelta must be non-negative');
+
+    _fracX += dx;
+    _fracY += dy;
+
+    int intX = _fracX.truncate();
+    int intY = _fracY.truncate();
+
+    if (intX == 0 && intY == 0) {
+      return null;
+    }
+
+    // Clamp before subtracting so excess movement is preserved in the accumulator
+    // rather than being permanently discarded during spikes.
+    if (maxDelta > 0) {
+      intX = intX.clamp(-maxDelta, maxDelta);
+      intY = intY.clamp(-maxDelta, maxDelta);
+    }
+
+    _fracX -= intX;
+    _fracY -= intY;
+
+    return RelativeMouseDelta(intX, intY);
+  }
+
+  void reset() {
+    _fracX = 0.0;
+    _fracY = 0.0;
+  }
+}