feat: add FullscreenButtonState to WebviewWindowOptions for API consistency

[leaanthony]

Description

Adds FullscreenButtonState ButtonState to WebviewWindowOptions for consistency with the existing MinimiseButtonState, MaximiseButtonState, and CloseButtonState fields.

Previously, the fullscreen button used setFullscreenButtonEnabled(bool) with a simple bool, while all other titlebar buttons used the ButtonState type (Enabled/Disabled/Hidden).

Changes:

• Add FullscreenButtonState ButtonState to WebviewWindowOptions
• Update macOS C code from setFullscreenButtonEnabled(bool) to setFullscreenButtonState(int) using the same setButtonState pattern as other buttons — now supports Hidden state too
• Add setFullscreenButtonState(ButtonState) to all platform implementations (Linux, Windows, iOS, Android)
• Apply FullscreenButtonState during window initialization on macOS and Windows
• Keep setFullscreenButtonEnabled(bool) for backward compatibility

Files changed:

• webview_window_options.go — new option field
• webview_window_darwin.go — C code + Go wrapper updated
• webview_window_linux.go — new stub
• webview_window_windows.go — new stub + init
• webview_window_ios.go — new stub
• webview_window_android.go — new stub

Fixes #4967

Type of change

• Enhancement (non-breaking change — new option field, default zero-value is ButtonEnabled which matches current behavior)

How Has This Been Tested?

• go build ./pkg/application/ compiles cleanly on Linux
• Cross-platform stubs verified for all platforms
• macOS testing needed — I don't have access to a macOS environment to verify the CGo changes

Checklist:

• My code follows the general coding style of this project
• My changes generate no new warnings
• Platform testing needed on macOS

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