Fix pointer style crash during early theme changes

[deepakganesh78]

Fixes #6360

Reproduction

A minimal app that sets pointer: default in CSS and switches to textual-light in __init__ crashes during startup because CSS is refreshed before any screen has been pushed.

Root cause

Updating a pointer style calls obj.node.screen.update_pointer_shape(). For the App node during early theme application, App.screen raises ScreenStackError because the screen stack is still empty; the pointer-style code only handled NoScreen.

Fix

Treat ScreenStackError like NoScreen for pointer updates, deferring pointer-shape refresh until a screen exists. This preserves existing pointer updates once the app is mounted.

Compatibility notes

This only suppresses an early pointer-shape refresh when no active screen exists. Existing validation and mounted pointer-style behavior are unchanged.

Validation

• python -m pytest tests\test_app.py::test_app_pointer_style_with_theme_before_screen_stack tests\test_app.py::test_pointer_shape -q → 2 passed
• Verified regression: with the source fix reverted, tests\test_app.py::test_app_pointer_style_with_theme_before_screen_stack fails with ScreenStackError: No screens on stack.
• python -m pytest tests --ignore=tests\snapshot_tests -q → 3013 passed, 1 skipped, 4 xfailed