fix: annotation form persists when selection is lost
The annotation box in the sidebar now stays visible when the user clicks
elsewhere on the page (losing text selection). This prevents the form from
disappearing unexpectedly when clicking buttons or composing notes.
Behavior:
- New valid selection: form updates with new selection
- Selection cleared (null): form keeps old selection (sticky)
- Clear button clicked: form explicitly closes
- Save successful/failed: existing behavior preserved
Added unit tests for selection persistence and E2E tests for annotation
form behavior.