Upstream UI kit and starter configuration fixes

[wkirby]

Summary

• upstream the Phlex/Tailwind UI kit from
• add the development-only UI kit gallery and component specs
• upstream shared RSpec, test-environment, lint-staged, Husky, and formatting configuration
• fix the starter's Didact app glob type so TypeScript checks pass

Validation

• docker compose run --rm -e RAILS_ENV=test web sh -lc 'bin/rails db:create db:schema:load && bundle exec rspec' (12 examples, 0 failures, 2 pending)
• bundle exec rubocop ... (UI kit and test configuration)
• npm run check
• npm run lint
• npm run prettier:check
• npx vite build

[egreer]

Should this be part of the starter or its own gem?