please fix examples (custom_window_frame)

**Describe the bug**
when using the custom_window_frame example there is some bugs specifically with the corner radius. after window is maximized then restored there is black color in the corner but it should be transparent. also when window is maximized it should not have corners and it also has border.

**To Reproduce**
Steps to reproduce the behavior:
cargo run --example custom_window_frame

**Expected behavior**

when window is maximized it should not have corner radius or border (im not talking about the actual gray border).
and when window is restored it should have corner radius with shadow and no fill color in corner.

**Screenshots**
what we see when app first start (and how it should look like after restoring from maximized):

maximized:

restored after maximized:

**Desktop:**
- OS: Windows 11
- Version 24H2 (26100.4652)

Start by running `cargo run --example custom_window_frame` on Windows 11 and inspect the custom_window_frame example. Reproduce maximizing and restoring the window, then trace how its corner radius, border, shadow, and corner fill are handled. Done means maximized windows have no radius or extra border, while restored windows regain rounded corners and shadow with transparent corners.

Written by the indexing model from the issue text.