sidecar: build FIPS from source in Docker, handle fipstop terminal init

- Add multi-stage Dockerfile: Rust builder stage compiles FIPS with
  --no-default-features --features tui (excludes BLE/libdbus)
- Move build context to repo root so Dockerfile can access Cargo.toml/src
- Add .dockerignore to exclude target/, .git/, testing/ from context
- Fix UDP port mapping to 2121:2121
- Delete scripts/build.sh cross-compile workflow
- Simplify README setup steps (no local Rust toolchain needed)
- Replace ratatui::init() with try_init() for clean error on terminal
  init failure (e.g. Docker on macOS Sequoia)

src/bin/fipstop/main.rs

@@ -121,7 +121,10 @@ fn main() {
         .expect("failed to create tokio runtime");
 
     let client = ControlClient::new(&socket_path);
-    let mut terminal = ratatui::init();
+    let mut terminal = ratatui::try_init().unwrap_or_else(|e| {
+        eprintln!("fipstop: failed to initialize terminal: {e}");
+        std::process::exit(1);
+    });
     let mut app = App::new(refresh);
     let events = EventHandler::new(refresh);