feat: add architecture docs center

[leoncheng57]

Summary

• add an in-app /docs engineering hub with a responsive architecture map
• render an explicit catalogue of canonical repository Markdown at /docs/:slug
• add architecture documentation, navigation, command-palette access, and mobile-safe styling
• extend deterministic PR screenshot and browser coverage for docs routes

Verification

• npm run typecheck
• npm test (295 passed)
• npm run build
• npm run test:e2e (163 passed, 1 expected screenshot-runner skip)
• npm run screenshots:local

full:/docs?directory=/tmp/mock-project
full:/docs/architecture?directory=/tmp/mock-project