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

src/common.rs

@@ -1,6 +1,6 @@
 mod components;
 mod ipc;
-mod localhost;
+pub(crate) mod localhost;
 mod message_loop;
 
 pub use components::*;