chore(11-01): add ANALYZE=true opt-in for rollup-plugin-visualizer in vite.config.js

- Wrapped config in async defineConfig factory so visualizer can be dynamically
  imported only when ANALYZE=true; normal builds incur zero overhead
- Added frontend/stats.html to .gitignore (ephemeral artifact; canonical copy
  lives in .planning/perf/)

.gitignore

@@ -5,4 +5,5 @@ backend/data/
 frontend/node_modules/
 frontend/dist/
 frontend/package-lock.json
+frontend/stats.html
 screenshots/