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

README.md

@@ -0,0 +1,43 @@
+# upl
+
+`upl` is a small personal resumable upload service. The intended deployment is:
+
+```text
+browser -> nginx -> upl Rust server -> local filesystem
+```
+
+The first implementation milestone provides the Rust server shell and static
+browser UI. Upload metadata, chunk persistence, resume state, and completion
+assembly are tracked in `PLAN.md` and will be added in later coherent slices.
+
+## Project Structure
+
+```text
+upl
+  Rust server
+    src/main.rs        binary entrypoint and listener setup
+    src/app.rs         Axum router, health endpoint, static file service
+    src/lib.rs         library surface used by integration tests
+  Browser UI
+    static/index.html  upload tool markup
+    static/styles.css  responsive tool styling
+    static/app.js      file-selection behavior
+  Validation
+    tests/             integration tests for server behavior
+    TESTS.md           reusable manual and automated test checklist
+```
+
+## Configuration
+
+- `UPL_BIND` sets the listen address. It defaults to `127.0.0.1:3000`.
+- `UPL_STATIC_DIR` sets the static asset directory. It defaults to `static/`
+  inside this repository.
+
+## Common Commands
+
+Use the `justfile` for routine tasks:
+
+```sh
+just check
+just run
+```