perf(frontend): defer mermaid and split editor dependencies

frontend/vite.config.ts

@@ -1,6 +1,18 @@
 import { defineConfig } from 'vite'
 import vue from '@vitejs/plugin-vue'
 import path from 'node:path'
+import { readFileSync } from 'node:fs'
+
+const mathChunks = new Map<string, string>()
+function mathChunk(module: string) {
+  const marker = '/node_modules/katex/'
+  const root = module.slice(0, module.lastIndexOf(marker) + marker.length)
+  if (!mathChunks.has(root)) {
+    const { version } = JSON.parse(readFileSync(`${root}package.json`, 'utf8'))
+    mathChunks.set(root, `math-katex-${version.replace(/[^0-9a-z]/gi, '-')}`)
+  }
+  return mathChunks.get(root)!
+}
 
 export default defineConfig({
   plugins: [vue()],
@@ -9,6 +21,23 @@ export default defineConfig({
       '@': path.resolve(__dirname, 'src'),
     },
   },
+  build: {
+    manifest: true,
+    rollupOptions: {
+      output: {
+        // Keep lazy languages/diagrams independent; do not collect every vendor into one bundle.
+        onlyExplicitManualChunks: true,
+        manualChunks(id) {
+          const module = id.replace(/\\/g, '/')
+          if (!module.includes('/node_modules/')) return
+          if (/\/@codemirror\/(?:state|view|language|commands|search|autocomplete|lint)\//.test(module) || /\/@lezer\/(?:common|highlight|lr)\//.test(module) || module.includes('/node_modules/codemirror/')) return 'editor-codemirror'
+          if (/\/node_modules\/prosemirror-[^/]+\//.test(module)) return 'editor-prosemirror'
+          if (module.includes('/node_modules/@milkdown/')) return 'editor-milkdown'
+          if (module.includes('/node_modules/katex/')) return mathChunk(module)
+        },
+      },
+    },
+  },
   server: {
     host: '127.0.0.1',
     port: 5173,