fix tsconfig and add typescript to dev dependencies

package.json

@@ -9,6 +9,7 @@
     },
     "devDependencies": {
         "sass": "~1.76.0",
+        "typescript": "^5.8.3",
         "vite": "^5.4.14",
         "vite-plugin-compression2": "^1.1.1"
     }

tsconfig.json

@@ -5,7 +5,8 @@
         "lib": ["es2021", "dom", "dom.iterable"],
         "strict": false, // TODO: fix and enable strict checking
         "moduleResolution": "node",
-        "skipLibCheck": true
+        "skipLibCheck": true,
+        "useDefineForClassFields": false
     },
-    "include": ["frontend/code/components/customTreeItem.ts"]
+    "include": ["frontend/code/**/*.ts"]
 }