Add test infrastructure and initial test suite

Sets up Vitest + React Testing Library with jsdom, exports utility functions
for unit testing, and adds 78 tests covering pure functions and component
integration flows (chapter loading, chunk management, navigation, Greek word
lookup).

https://claude.ai/code/session_01C4T3bYHR2svLq7JVcxKJ6Y

src/test-setup.js

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