Use fake-indexeddb to provide IndexedDB API in Node.js test environment.
This fixes 10 failing tests in spellbook-storage.test.ts that were
previously blocked by missing IndexedDB.
Changes:
- Add fake-indexeddb as dev dependency
- Create vitest setup file that imports the polyfill
- Update vitest.config.ts to use setup file
- Fix relay-selection.test.ts to clear cache between tests for isolation
