feat: add webview-only transparency support for macOS

[clearlysid]

Add make_webview_transparent() method that makes only the webview background transparent while keeping the window opaque, providing more granular control over transparency.

• Add make_webview_transparent() method to WebviewWindowExt trait
• Implement webview-only transparency using NSView drawsBackground property
• Update example app to demonstrate webview transparency
• Add transparent CSS styling to showcase the effect

🤖 Generated with Claude Code

[Copilot]

Pull Request Overview

This PR introduces a new API to make only the webview background transparent on macOS while keeping the window decorations opaque, and updates the example app to showcase this feature.

• Add make_webview_transparent() to the WebviewWindowExt trait and implement it using Cocoa’s KVC.
• Update example application’s CSS and React component to demonstrate transparent webview regions.
• Adjust the macOS window setup in main.rs to call the new API and tweak traffic light insets.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/lib.rs	Defines and implements make_webview_transparent() in the trait, disabling the webview’s drawsBackground.
examples/tauri-app/src/styles.css	Sets the app background to transparent and adds .transparent-demo styles for the demo section.
examples/tauri-app/src/App.tsx	Inserts a new .transparent-demo block in the UI to showcase the transparency effect.
examples/tauri-app/src-tauri/src/main.rs	Wraps the traffic-light inset call in a cfg block and invokes make_webview_transparent() on macOS.

Comments suppressed due to low confidence (2)

src/lib.rs:217

• [nitpick] The variable name id is ambiguous and shadows the Cocoa id type alias. Rename it to something like view_ptr or webview_obj for clarity.

            let id = webview.inner() as *mut objc::runtime::Object;

src/lib.rs:209

• This new make_webview_transparent method currently has no tests. Consider adding a unit or integration test (perhaps via a headless Cocoa environment or mocking) to validate that drawsBackground is toggled as expected.

    fn make_webview_transparent(&self) -> Result<&WebviewWindow, Error> {