test(schemas): replace CNA-era react/tailwind fixtures with CPA domain types

[ulises-jeremias]

Context

src/lib/__tests__/schemas.test.ts still uses CNA-era fixtures (type: 'react', Tailwind, "Frontend Applications"). That confuses CPA contributors and weakens the test as documentation.

Steps

1. Read canonical types/slugs from Create-Python-App/cpa-templates/templates.json.
2. Rewrite fixtures to CPA domain types (fastapi-backend, django-backend, cli-app, etc.).
3. Use real extension slugs such as fastapi-docker / github-setup.
4. Run pnpm test in the website repo.

Acceptance criteria

• No React/Tailwind frontend fixtures remain
• Category/type/extension examples match CPA registry vocabulary
• All vitest schema tests pass
• Diff is limited to test fixtures (unless a schema bug is found)

Files

• src/lib/__tests__/schemas.test.ts
• src/lib/schemas.ts (reference)
• cpa-templates/templates.json (reference)

How to verify

pnpm test (or the repo's documented test script) passes.