Add Svelte GUI, FIT encoder and README

Standalone binary embeds the frontend, avoiding the dev-server dependency
that made the window fail to load.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -75,6 +75,12 @@ pub fn run() {
             handle.state::<AppState>().lock().devices.start_scan();
             state::spawn_ride_loop(handle.clone());
             state::spawn_device_loop(handle.clone());
+            // `BIKECONTROL_DEMO=1` opens straight onto a running ride with the
+            // bundled GPX loaded. Purely a development convenience — it makes
+            // the ride screen reviewable without clicking through first.
+            if std::env::var("BIKECONTROL_DEMO").is_ok() {
+                state::start_demo(&handle);
+            }
             state::emit_devices(&handle);
             state::emit_ride_state(&handle);
             Ok(())