Set up unit testing infrastructure

- Extract slot generation logic into apps/api/src/lib/slots.ts for testability
- Add 8 unit tests covering slot generation edge cases (overlap, multi-groomer, boundary)
- Add @testing-library/react + jsdom to apps/web; configure vitest with jsdom environment
- Add 4 component tests for App navigation rendering and active-link highlighting
- Remove passWithNoTests: true from both vitest configs; add coverage thresholds

Closes #39

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/src/test/setup.ts

@@ -0,0 +1 @@
+import "@testing-library/jest-dom";