Set up unit testing infrastructure

Extract slot generation from book.ts into pure utility for unit testing.
Add 8 API unit tests and 4 web component tests with 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";