feat: create highlight manager with IndexedDB storage

Implemented TDD approach for highlight persistence:
- Created IndexedDB store with 'highlights' object store
- Added indexes for syncStatus and verseId for efficient queries
- Implemented CRUD operations: add, update, get, getAll, delete
- Added query methods: getHighlightsByVerse, getPendingHighlights
- Full test coverage with fake-indexeddb mock
- Added structuredClone polyfill for test environment

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

jest.setup.js

@@ -0,0 +1,7 @@
+import '@testing-library/jest-dom'
+import 'fake-indexeddb/auto'
+
+// Polyfill for structuredClone (required by fake-indexeddb)
+if (typeof global.structuredClone === 'undefined') {
+  global.structuredClone = (obj) => JSON.parse(JSON.stringify(obj))
+}