chore: stop tracking manifest.json (generated at build time)

The manifest.json is regenerated by scripts/generate-manifest.js during
the prebuild process. It should not be tracked in git.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: TortoiseWolfe

.gitignore

@@ -17,6 +17,9 @@
 /.next/
 /out/
 
+# generated files (created by prebuild scripts)
+/public/manifest.json
+
 # production
 /build