feat:Add opt‑in navigation handler for non‑dioxus:// URLs in `dioxu…

[k5602]

This PR adds an optional navigation callback to dioxus_desktop::Config so applications can decide whether to allow non‑dioxus:// navigations inside the WebView. Default behavior is unchanged.

Motivation

dioxus-desktop currently blocks all non‑dioxus:// navigations and opens them externally. That prevents legitimate in‑app flows (e.g., localhost relays for embedded content). The only workaround today is vendoring/patching the crate. This PR provides a clean, opt‑in API.

What’s Included

• Config::with_navigation_handler(|url: &str| -> bool)
• Stored in Config as navigation_handler: Option<…>
• Integrated into WebViewBuilder::with_navigation_handler:
  • If set, the callback decides whether a URL is allowed in‑webview.
  • Otherwise current behavior remains unchanged.

Behavior (unchanged by default)

• No handler → same current rules, same external navigation behavior.
• Handler provided → app decides for non‑dioxus:// URLs.

Example Usage

use dioxus_desktop::Config;

let cfg = Config::new().with_navigation_handler(|url| {
    url.starts_with("http://127.0.0.1:")
});

Files Changed

• packages/desktop/src/config.rs
• packages/desktop/src/webview.rs

Notes

• No breaking changes.
• Keeps existing security posture by default.
• Enables controlled local or trusted navigations for desktop apps.