Fix esbuild vulnerability by upgrading Vite 5 → 6 and vitest

Update vite 5.4.21 → 6.4.1, @vitejs/plugin-vue and vitest to resolve
Dependabot alert #1 (esbuild dev server request forgery, now at 0.25.12)

frontend/package.json

@@ -16,6 +16,7 @@
     "format:check": "prettier --check src/"
   },
   "dependencies": {
+    "@vitest/mocker": "^4.1.2",
     "dompurify": "^3.3.3",
     "marked": "^17.0.4",
     "pinia": "^2.3.0",
@@ -25,14 +26,14 @@
   "devDependencies": {
     "@eslint/js": "^9.0.0",
     "@types/dompurify": "^3.2.0",
-    "@vitejs/plugin-vue": "^5.2.0",
+    "@vitejs/plugin-vue": "^6.0.5",
     "eslint": "^9.0.0",
     "eslint-plugin-vue": "^9.32.0",
     "prettier": "^3.4.0",
     "typescript": "^5.9.3",
     "typescript-eslint": "^8.57.1",
-    "vite": "^5.4.0",
-    "vitest": "^2.1.0",
+    "vite": "^6.4.1",
+    "vitest": "^4.1.2",
     "vue-tsc": "^2.2.12"
   }
 }