Fix NaN% in sidecar download progress

The Rust backend emits {downloaded, total, phase, message} but the
Svelte component was reading event.payload.progress which doesn't
exist, resulting in NaN. Now calculates percentage from downloaded/total.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/components/SidecarSetup.svelte

@@ -36,11 +36,12 @@
 
     try {
       // Listen for progress events from the Tauri backend
-      unlisten = await listen<{ progress: number; message: string }>(
+      unlisten = await listen<{ downloaded: number; total: number; phase: string; message: string }>(
         "sidecar-download-progress",
         (event) => {
-          progress = event.payload.progress;
+          const { downloaded, total, message } = event.payload;
-          progressMessage = event.payload.message;
+          progress = total > 0 ? (downloaded / total) * 100 : 0;
+          progressMessage = message;
         }
       );