⬆️(frontend) replace NodeJS.Timeout with ReturnType<typeof setTimeout>

Removes Node.js type dependency from browser/React code by substituting
the non-portable `NodeJS.Timeout` type with the standard
`ReturnType<typeof setTimeout>` equivalent across all affected files.

Also replaces `process.env.NODE_ENV` in Icon.tsx with
`import.meta.env.MODE` for Vite compatibility.

src/frontend/package-lock.json

@@ -44,7 +44,6 @@
         "@tanstack/eslint-plugin-query": "5.100.14",
         "@tanstack/react-query-devtools": "5.100.14",
         "@types/humanize-duration": "3.27.4",
-        "@types/node": "24.12.4",
         "@types/react": "18.3.12",
         "@types/react-dom": "18.3.1",
         "@vitejs/plugin-react": "6.0.2",
@@ -58,7 +57,7 @@
         "postcss": "8.5.15",
         "prettier": "3.8.3",
         "rollup-plugin-visualizer": "7.0.1",
-        "typescript": "5.8.3",
+        "typescript": "6.0.3",
         "typescript-eslint": "8.60.1",
         "vite": "8.0.14",
         "vite-plugin-svgr": "5.2.0",
@@ -12611,9 +12610,9 @@
       }
     },
     "node_modules/typescript": {
-      "version": "5.8.3",
-      "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.8.3.tgz",
-      "integrity": "sha512-p1diW6TqL9L07nNxvRMM7hMMw4c5XOo/1ibL4aAIGmSAt9slTE1Xgw5KWuof2uTOvCg9BY7ZRi+GaF+7sfgPeQ==",
+      "version": "6.0.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-6.0.3.tgz",
+      "integrity": "sha512-y2TvuxSZPDyQakkFRPZHKFm+KKVqIisdg9/CZwm9ftvKXLP8NRWj38/ODjNbr43SsoXqNuAisEf1GdCxqWcdBw==",
       "devOptional": true,
       "license": "Apache-2.0",
       "bin": {