fix(windows): use webview2_com built-in handler instead of #[implement]

The previous approach used windows::core::implement to manually implement
ICoreWebView2PermissionRequestedEventHandler_Impl, but the sealed
IUnknownImpl constraint in webview2-com prevents external #[implement]
usage — you must use the crate's own pre-built handler types.

Replace with PermissionRequestedEventHandler::create() which wraps the
closure and handles COM ref-counting internally. Also removes the now-
unused windows = "0.61" direct dependency.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -44,7 +44,6 @@ tauri-plugin-updater = "2"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 webview2-com = "0.38"
-windows = { version = "0.61", features = [] }
 
 [lib]
 name = "app_lib"