`template-vercel`: Never-expire sandbox snapshots and simplify runtime code

- Upgrade @vercel/sandbox from 1.3.1 to 1.6.0
- Pass `{ expiration: 0 }` to `sandbox.snapshot()` for infinite retention
- Remove expiration tracking from snapshot cache
- Replace `reuseOrCreateSandbox` with simpler `getSandbox` (no runtime fallback)
- Remove `subtitle` field from progress types, state, and UI
- Run Prettier on src

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>