Summary

• Adds Page.theme_animation_style: Optional[AnimationStyle], exposing Flutter's MaterialApp.themeAnimationStyle so apps can tune the duration/curve of the cross-fade between theme and dark_theme — or disable it entirely via AnimationStyle.no_animation().
• Wires the property through both MaterialApp(...) and MaterialApp.router(...) constructions in packages/flet/lib/src/controls/page.dart (CupertinoApp branches are unchanged — Flutter has no equivalent there).
• Adds a showcase example under sdk/python/examples/controls/core/types/theme_animation_style/showcase/ that lets the user pick between Default, Custom (2 s easeInOutCubic), and None styles, with a "Switch Theme Mode" button that toggles light/dark so the chosen transition is observable.

Test plan

• Run the new example: cd sdk/python/examples/controls/core/types/theme_animation_style/showcase && flet run main.py
• Select Default → click "Switch Theme Mode" → expect the standard short cross-fade
• Select Custom → toggle theme → expect a clearly slower (~2 s) cubic-eased fade
• Select None → toggle theme → expect an instantaneous swap with no fade
• Verify rapid toggling during an in-flight Custom transition behaves sensibly (no crash)
• Confirm an unrelated theme example (e.g. core/types/theme_mode/showcase) still runs unchanged with theme_animation_style defaulting to None

Summary by Sourcery

Expose configurable theme animation style on Page and showcase its usage.

New Features:

• Add Page.theme_animation_style property to control theme transition animations between light and dark modes.
• Support forwarding theme_animation_style to underlying Flutter MaterialApp and MaterialApp.router constructors.
• Introduce a new Python example showcasing default, custom, and disabled theme animation styles for theme switching.

Documentation:

• Document Page.theme_animation_style usage and its ability to disable theme transition animations in the Python API docstring.

Tests:

• Add a gallery/showcase example project configuration for theme animation style to aid manual testing and demonstration.