refactor (window): replace raw isize window handles with self-documenting Window and custom ID types

[JustForFun88]

This pull request makes the hwnd field of the Window struct private and updates the codebase to use the explicit Window type or dedicated ID structs (such as BorderId, StackbarId) instead of raw isize handles. This makes all window management logic self-documenting, safer, and easier to maintain.

Motivation & Benefits:

• All data structures and APIs now use intention-revealing types instead of ambiguous integers (e.g., ReaperNotification(pub HashMap<Window, (usize, usize)>)).
• Removes isize identifiers from related structs, introducing clear ID types.
• Improves type safety and enforces encapsulation, making the API and overall codebase more robust and understandable.
• The code is now more self-documenting — window-related logic is explicit and easy to follow at a glance.

Key Refactorings:

• Hidden now holds a private Window field instead of isize.
• Stackbar now uses a StackbarId struct that wraps a Window.
• Border now stores a BorderId and tracking_window: Window instead of raw handles.

Other Notes:

• Serialization logic remains compatible with previous versions for backward compatibility.
• This PR is based on refactor(container + bm): use Arc<str> and WsElementId for Workspace Element IDs #1489 and refactor(ring): stop exposing Ring internals #1491.