feat: add reserve-grid shared route overlap

[iamasx]

Summary

• add the new reserve-grid route with zone-driven reserve cells and linked overflow surfaces
• rewrite the landing-page hero and shared app shell to feature Reserve Grid and reserve styling tokens
• align current scenario-board and field-guide tests with the shipped UI so lint, tests, and build pass on this branch

Verification

• npm run lint
• npm run test
• npm run build

Closes #219