Fix pointer style becoming sticky on screen removal

[veeceey]

Summary

Fixes #6349 - The mouse pointer (cursor) style becomes "sticky" when a modal screen containing a Button is dismissed. The cursor remains as a pointing-hand instead of reverting to the default arrow.

Root cause: Two issues contributed:

1. Stale mouse_over reference: update_pointer_shape() used app.mouse_over without checking whether the widget belonged to the current screen. After a screen dismiss, mouse_over could still reference a widget from the removed screen (e.g., a Button with pointer: pointer style), causing the stale pointer shape to persist.

2. Missing pointer refresh on screen resume: _on_screen_resume did not re-send the pointer shape to the terminal. Since _pointer_shape is a reactive var, the watcher only fires on value changes. If the resumed screen already had _pointer_shape="default", no escape sequence was emitted -- so the terminal kept showing the dismissed screen's pointer shape.

Fix:

• Guard update_pointer_shape() against stale cross-screen mouse_over references by checking if the widget is an ancestor of the current screen.
• Force-send the pointer shape escape sequence in _on_screen_resume after recalculating the correct shape, regardless of whether the reactive value changed.

Test plan

• Added regression test test_pointer_shape_restored_on_screen_dismiss that reproduces the exact scenario from the issue (push modal with Button, hover over button, dismiss, verify pointer resets)
• Existing test_pointer_shape test continues to pass
• All test_app.py tests pass (27/27)
• All test_screens.py and test_modal.py tests pass (20/20)

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