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/state.rs

@@ -236,6 +236,30 @@ fn transmit(_app: &AppHandle, target: ControlTarget) {
     tracing::debug!(?target, "control target (no trainer attached — mock backend)");
 }
 
+/// Load the bundled GPX and start riding it. Development only — see the
+/// `BIKECONTROL_DEMO` check in `lib.rs`.
+pub fn start_demo(app: &AppHandle) {
+    let Some(sample) = crate::samples::all().into_iter().find(|s| s.is_gpx) else {
+        return;
+    };
+    let profile = match bikecontrol_core::gpx::import(
+        &sample.text,
+        &sample.name,
+        &bikecontrol_core::gpx::SmoothingConfig::default(),
+    ) {
+        Ok(p) => p,
+        Err(e) => {
+            tracing::warn!(%e, "demo profile failed to import");
+            return;
+        }
+    };
+    let (view, geom) = crate::profile_view::build(&profile, "demo");
+    let state = app.state::<AppState>();
+    let mut inner = state.lock();
+    inner.set_profile(profile, view, geom);
+    inner.inputs.status = RideStatus::Running;
+}
+
 /// SAF-2: never leave the trainer loaded. Called on ride end and on app exit.
 pub fn release_trainer(app: &AppHandle) {
     let state = app.state::<AppState>();