feat(web): SPA skeleton embedded in the binary (#60)

crates/arr-daemon/src/main.rs

@@ -1,6 +1,7 @@
 //! arr — reconcile loop and process entry point. See DESIGN.md §8.
 
 mod config;
+mod web;
 
 use std::process::ExitCode;
 use std::sync::Arc;
@@ -94,6 +95,7 @@ async fn run() -> Result<(), Error> {
 
     let app = arr_api::router(state)
         .merge(arr_compat::router(compat))
+        .fallback(web::serve)
         .layer(TraceLayer::new_for_http());
 
     let listener = tokio::net::TcpListener::bind(config.bind_addr)