feat: unify webview source into WebviewSource enum with dynamic reload (#22)

Unify CefWebviewUri and InlineHtml into a single WebviewSource enum component, eliminating the dual-component inconsistency where both existed on the same entity
Add ResolvedWebviewUri internal component as a resolution layer between user-facing source and CEF
Support dynamic reload: mutating WebviewSource at runtime automatically navigates the existing browser via browsers.navigate() without recreation
Inline HTML is served via cef://localhost/__inline__/{id} with automatic cleanup on entity despawn

examples/navigation.rs

@@ -50,7 +50,7 @@ fn spawn_webview(
 ) {
     commands.spawn((
         DebugWebview,
-        CefWebviewUri::new("https://github.com/not-elm/bevy_cef"),
+        WebviewSource::new("https://github.com/not-elm/bevy_cef"),
         Mesh3d(meshes.add(Plane3d::new(Vec3::Z, Vec2::ONE))),
         MeshMaterial3d(materials.add(WebviewExtendStandardMaterial::default())),
     ));