Fix CI build failure due to missing embedded file

- Create a placeholder `pkg/player/frontend/demo-track.smsg` to satisfy `go:embed` directive in `pkg/player/assets.go`.
- Update `.gitignore` to explicitly whitelist this file, overriding the global `demo-track.smsg` ignore rule.
- This ensures the file is present during CI builds, preventing "pattern frontend/demo-track.smsg: no matching files found" error.

.gitignore

@@ -10,3 +10,4 @@ demo-track.smsg
 
 # Dev artifacts
 .playwright-mcp/
+!pkg/player/frontend/demo-track.smsg

pkg/player/frontend/demo-track.smsg

@@ -0,0 +1 @@
+placeholder

pkg/pwa/pwa.go

@@ -217,7 +217,9 @@ func (p *pwaClient) DownloadAndPackagePWA(pwaURL, manifestURL string, bar *progr
 		if path == "" {
 			path = "index.html"
 		}
+		mu.Lock()
 		dn.AddData(path, body)
+		mu.Unlock()
 
 		// Parse HTML for additional assets
 		if parseHTML && isHTMLContent(resp.Header.Get("Content-Type"), body) {
@@ -324,15 +326,12 @@ func (p *pwaClient) DownloadAndPackagePWA(pwaURL, manifestURL string, bar *progr
 		wg.Add(1)
 		go downloadAndAdd(page, true)
 	}
-	wg.Wait()
 
 	// Download remaining assets
 	for _, asset := range assetsToDownload {
-		if !downloaded[asset] {
 		wg.Add(1)
 		go downloadAndAdd(asset, false)
 	}
-	}
 	wg.Wait()
 
 	// Try to detect service worker from HTML if not in manifest