feat(desktop): expose navigation_handler in Config

[FreeSynergy]

Summary

The navigation handler in dioxus-desktop is currently hardcoded in webview.rs:

.with_navigation_handler(|var| {
    if var.starts_with("dioxus://") || var.starts_with("http://dioxus.") {
        true
    } else {
        if var.starts_with("http://") || var.starts_with("https://") {
            _ = webbrowser::open(&var);
        }
        false
    }
})

This unconditionally opens every external http/https URL in the system browser and blocks it in the WebView. Apps that intentionally embed web content (e.g. via <iframe>) have no way to opt out of this behavior or to allow specific URLs.

Changes

• config.rs: adds navigation_handler: Option<Box<dyn Fn(String) -> bool + 'static>> field to Config, initialized to None.
• config.rs: adds pub fn with_navigation_handler(mut self, handler: impl Fn(String) -> bool + 'static) -> Self builder method with full doc-comment and example.
• webview.rs: if cfg.navigation_handler is Some, the closure delegates entirely to the user-supplied handler; otherwise the existing hardcoded behavior is preserved exactly.

Non-breaking

When with_navigation_handler is not called (the default), behavior is identical to before — no existing apps are affected.

Example

let cfg = Config::new()
    .with_navigation_handler(|url| {
        // Allow dioxus internal URLs and a trusted external origin
        url.starts_with("dioxus://") || url.starts_with("https://trusted.example.com")
    });