win32: fix WM_TASKBARCREATED deadlock

[Invades]

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality

Fixes a Win32 UI-thread deadlock when handling the registered TaskbarCreated message.
The handler previously held window_state_lock() while calling set_skip_taskbar(...). That call can synchronously re-enter the same window proc through Win32, and the re-entered path can try to acquire the same window state lock again, deadlocking the UI thread.
This copies skip_taskbar out of the window state first, so the lock is dropped before calling set_skip_taskbar(...).

No changelog/docs/example added because this is an internal Win32 deadlock fix with no user-facing API or behavior change beyond avoiding the hang.