feat(router): add opt-in View Transition support to Link

[rexlunae]

Closes #2658

Adds a view_transition prop (default false, opt-in) to the router's Link component. When enabled, clicking the link wraps the navigation in the browser's View Transition API (document.startViewTransition), animating the route change. This is a no-op on browsers/webviews that don't support the API.

How it works
The click handler calls document.startViewTransition(() => {}) via dioxus_document::eval before router.push_any. The old page state is captured synchronously at call time; Dioxus applies the DOM mutations in the microtask checkpoint right after the event handler; the browser captures the new state on the next frame — inside the transition.

Notes

• Works on web and desktop (wry webview); JS guard makes it a no-op where unsupported.
• SSR is unaffected: the click handler never runs server-side, and dioxus-document's no-op document would only produce an async EvalError::Unsupported that is never awaited.
• The dioxus-document dependency is optional and tied to the existing html feature, matching dioxus-html.

Testing

• cargo test -p dioxus-router: all pass (51 tests)
• cargo clippy -p dioxus-router: clean
• cargo fmt -p dioxus-router --check: clean
• Not manually browser-tested in this environment — would appreciate a dx serve sanity check.

cc @ealmloff @jkelleyrtp