GRAL-5705: Downgrade Next.js to 13.5.11 for Node 16 compatibility

The initial commit upgraded Next.js from 13.5.1 to 15.0.8, but Next.js 15
requires Node.js 18+. Downgraded to 13.5.11 (latest 13.x) which:
- Maintains compatibility with Node.js 16
- Includes security patches from 13.5.1 → 13.5.11
- Avoids breaking changes from Next.js 14/15

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: IgnorancePulls