test(editor): add tests for notes/highlights system changes
Add unit tests for highlight serialization/deserialization with empty text
fields, line-anchored highlights, round-trip validation, and edge cases.
Add Playwright tests for:
- Cmd+Shift+H toggles notes pane
- Cmd+Shift+L with selection creates highlight with empty text
- Cmd+Shift+L without selection anchors to current line
- Notes pane shows 'Line N — highlight' for empty notes
- Notes pane shows 'Line N' with note text for annotated highlights
- Notes pane has Edit button
- Cmd+Shift+L auto-opens notes pane
- Notes pane title says 'Notes'
Update test-layout-page.html with annotation CSS styles and test helpers
for programmatic highlight creation, selection, and inspection.