fix(title_bar): clear CSD drag flag via paint-phase window-level mous…

[gaoyia]

On Linux client-side decorations, when the pointer leaves the title-bar element mid-drag, the element-level on_mouse_up never fires. As a result should_move stays true, and the next mouse move re-triggers start_window_move, causing the window to keep moving unexpectedly.

This PR registers a window-level mouse_up listener that resets should_move. Because window.on_mouse_event may only be called during the paint phase (calling it from RenderOnce::render panics with "this method can only be called during paint"), the listener is wrapped in a zero-size Element (TitleBarWindowMouseUpListener) that is only attached on Linux when the window is client-decorated.

The existing drag and double-click logic is left unchanged.

Description

On Linux with client-side decorations (CSD), the title bar tracks a should_move flag: mouse-down sets it, the next mouse-move calls start_window_move, and mouse-up clears it. The problem is that once the compositor begins moving the window, the pointer can leave the title-bar element, so its element-level on_mouse_up never fires. The flag stays true, and a later mouse-move re-triggers start_window_move, making the window "stick" to the cursor.

This PR adds a window-level mouse_up listener that clears should_move regardless of where the pointer is released. Since window.on_mouse_event may only be called during the paint phase — calling it from RenderOnce::render panics with "this method can only be called during paint" — the listener is implemented as a zero-size Element (TitleBarWindowMouseUpListener, display: none) that registers the listener in its paint. The listener only reacts to left-button release during the bubble phase. It is attached only when is_linux && is_client_decorated, so other platforms and server-side decorations are unaffected. The existing drag and double-click (maximize) behavior is unchanged.

Break Changes

None.

How to Test

On Linux with client-side decorations:

1. Press and hold the left mouse button on the title bar and start dragging the window.
2. While dragging, move the pointer off the title-bar element (e.g. over the window content), then release the button.
3. Move the mouse again — the window should NOT resume following the cursor.

Also verify normal behavior is intact:

• Dragging the title bar moves the window.
• Double-clicking the title bar toggles maximize.

On macOS/Windows, confirm no behavioral change (the listener is not attached).

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)