feat: add capacity board overlap route

[iamasx]

Summary

• add a new /capacity-board route with pod load, overflow coverage, and linked route rails
• rewrite the landing page hero so the new route is linked from the main entry point
• update the shared layout shell and global styles to widen the intentional conflict surface

Closes #218

Verification

• npm run lint
• npm run build
• npm run test (fails in existing unrelated suites: src/app/scenario-board/page.test.tsx and src/app/field-guide/page.test.tsx)