Migrating from hover-based transparency to pointer-moving based trans…

[Lori-Shu]

…parency

[pullfrog]

✅ No new issues found.

Reviewed changes — this PR refactors UI transparency behavior by removing hover-based visibility triggers across BodyUI, ControlbarUI, and HeadbarUI, replacing them with a centralized detect_pointer_moving mechanism in AppUI.

• Centralized Logic: Implemented AppUI::detect_pointer_moving to manage visibility based on pointer movement, centralizing the responsibility rather than distributing it across sub-components.
• Cleanup: Removed visible_flag fields from BodyUI, ControlbarUI, and HeadbarUI structs, as they are no longer responsible for updating this state.

  ^{｜ View workflow run ｜ Using google/gemini-3.1-flash-lite ｜ 𝕏}