⬆️(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.json

@@ -51,7 +51,6 @@
     "@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",
@@ -65,7 +64,7 @@
     "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",