fix: three TypeScript build errors in frontend

- Remove unused onMounted import in VisualEditor.vue
- Add missing @codemirror/theme-one-dark to devDependencies
- Add vite-env.d.ts so import.meta.env is typed correctly

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -23,6 +23,7 @@
   },
   "devDependencies": {
     "@codemirror/lang-markdown": "^6.2.5",
+    "@codemirror/theme-one-dark": "^6.1.2",
     "@types/node": "^20.14.0",
     "@vitejs/plugin-vue": "^5.0.5",
     "autoprefixer": "^10.4.19",