perf: Vite 分包优化 — Element Plus / Vue 独立 chunk

build.rollupOptions.manualChunks:
- element-plus → ~923 kB (独立缓存，首次加载后 HIT)
- vue-vendor → ~110 kB (Vue + Router + Pinia)
- index (app code) → ~59 kB (从 ~1 MB 缩减)

首屏加载显著减少，二次访问几乎零加载。

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

frontend/vite.config.ts

@@ -9,6 +9,16 @@ export default defineConfig({
       '@': resolve(__dirname, 'src'),
     },
   },
+  build: {
+    rollupOptions: {
+      output: {
+        manualChunks: {
+          'element-plus': ['element-plus'],
+          'vue-vendor': ['vue', 'vue-router', 'pinia'],
+        },
+      },
+    },
+  },
   server: {
     port: 5173,
     proxy: {