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>

ui/src/lib/app.svelte.ts

@@ -48,6 +48,9 @@ class AppStore {
     this.ride = ride;
     this.devices = devices;
     this.samples = samples;
+    // A ride already in progress (a reload, or an autostart) belongs on screen
+    // immediately — nobody wants to click past a device list mid-effort.
+    if (ride.status === 'running' || ride.status === 'paused') this.screen = 'ride';
 
     await subscribe({
       onFrame: (f) => this.onFrame(f),