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/

frontend/vite.config.js

@@ -1,8 +1,23 @@
-import { defineConfig } from 'vite'
+import { defineConfig, loadEnv } from 'vite'
 import vue from '@vitejs/plugin-vue'
 
-export default defineConfig({
-  plugins: [vue()],
+// Bundle analyzer: only active when ANALYZE=true is set in the environment.
+// Usage: ANALYZE=true npm run build
+// Output: stats.html in the project root (committed to .planning/perf/ manually).
+function conditionalAnalyzer(env) {
+  if (env.ANALYZE !== 'true') return null
+  // Dynamic import so the package is never executed during normal builds.
+  return import('rollup-plugin-visualizer').then(({ visualizer }) =>
+    visualizer({ open: false, filename: 'stats.html', gzipSize: true })
+  )
+}
+
+export default defineConfig(async ({ mode }) => {
+  const env = loadEnv(mode, process.cwd(), '')
+  const analyzerPlugin = await conditionalAnalyzer(env)
+
+  return {
+    plugins: [vue(), ...(analyzerPlugin ? [analyzerPlugin] : [])],
     build: {
       // top-level await in main.js requires esnext target
       target: 'esnext',
@@ -17,4 +32,5 @@ export default defineConfig({
         },
       },
     },
+  }
 })