Upgrade to Next.js 16.1.6, React 19.2.4, and update all dependencies

- Next.js 15.5 → 16.1.6 (Turbopack default, faster builds)
- React/React-DOM 19.1 → 19.2.4
- @types/react 18 → 19, @types/react-dom 18 → 19
- TypeScript 5.2 → 5.9
- CI Node.js 18 → 20 (required by Next.js 16)
- Replace removed next lint with standalone ESLint config
- Auto-updated tsconfig.json (jsx: react-jsx, dev types)

.gitea/workflows/deploy.yml

@@ -45,7 +45,7 @@ jobs:
         if: steps.changes.outputs.frontend == 'true'
         uses: actions/setup-node@v4
         with:
-          node-version: 18
+          node-version: 20
 
       - name: Install dependencies
         if: steps.changes.outputs.frontend == 'true'