feat(extension): add e2e sync tests and preserve original timestamps
- Add comprehensive e2e tests for browser extension sync:
- Browser imports (history, bookmarks, tabs) stay local (syncSource prevents push)
- User-added items (notes, URLs) sync bidirectionally
- Deduplication behavior
- Tags and metadata handling
- Preserve original timestamps as createdAt during imports:
- History: uses earliest visit time
- Bookmarks: uses dateAdded
- User items: use current time (default)
- DataEngine.addItem() now accepts optional createdAt parameter for imports
Tests: 129 passing (all extension tests)