feat: serve static upload app from axum

Introduce the first PLAN.md milestone: replace the hello-world binary with
an Axum server that binds to localhost by default, exposes a health endpoint,
and serves the static browser UI from the repository's static directory. The
router is available through the library crate so integration tests can exercise
server behavior without opening a network listener.

Add a justfile for routine validation and document the initial project shape,
configuration knobs, and reusable test checklist. The rustfmt config now uses
only stable options so the new formatting recipe runs without nightly warnings.

The upload API and resumable chunk behavior are intentionally left for later
milestones; the UI currently handles file selection only.

Test Plan:
- just check

Refs: PLAN.md milestone 1

tests/static_server.rs

@@ -0,0 +1,48 @@
+use std::net::{Ipv4Addr, SocketAddr};
+
+use axum::{body::Body, http::Request};
+use http_body_util::BodyExt;
+use tower::ServiceExt;
+use upl::app::{AppConfig, build_router};
+
+#[tokio::test]
+async fn serves_index_page() -> Result<(), Box<dyn std::error::Error>> {
+    let app = test_app();
+
+    let response = app
+        .oneshot(Request::builder().uri("/").body(Body::empty())?)
+        .await?;
+
+    assert_eq!(response.status(), axum::http::StatusCode::OK);
+
+    let body = response.into_body().collect().await?.to_bytes();
+    let body = String::from_utf8(body.to_vec())?;
+
+    assert!(body.contains("<title>upl</title>"));
+    assert!(body.contains("Select file"));
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn reports_health() -> Result<(), Box<dyn std::error::Error>> {
+    let app = test_app();
+
+    let response = app
+        .oneshot(Request::builder().uri("/healthz").body(Body::empty())?)
+        .await?;
+
+    assert_eq!(response.status(), axum::http::StatusCode::OK);
+
+    let body = response.into_body().collect().await?.to_bytes();
+    assert_eq!(body.as_ref(), b"ok");
+
+    Ok(())
+}
+
+fn test_app() -> axum::Router {
+    build_router(&AppConfig::new(
+        SocketAddr::from((Ipv4Addr::LOCALHOST, 0)),
+        concat!(env!("CARGO_MANIFEST_DIR"), "/static"),
+    ))
+}