fix(ci): add placeholder for missing demo-track.smsg

The build was failing because `pkg/player/assets.go` embeds `frontend/demo-track.smsg`,
which was ignored by git and missing in the repository checkout.

This commit:
- Adds a placeholder `pkg/player/frontend/demo-track.smsg` file.
- Updates `.gitignore` to explicitly track this specific file while keeping the global ignore rule.

This ensures `go:embed` can find the file during build/test in CI environments.

.gitignore

@@ -7,6 +7,7 @@ coverage.txt
 
 # Demo content (hosted on CDN)
 demo-track.smsg
+!pkg/player/frontend/demo-track.smsg
 
 # Dev artifacts
 .playwright-mcp/

pkg/player/frontend/demo-track.smsg

@@ -0,0 +1 @@
+placeholder for build