Add suggestion state when no focus is set but items exist in queue:
- FocusView now shows SuggestionCard when current is null but queue has items
- SuggestionCard displays suggested item with 'Set as focus' button
- Clicking 'Set as focus' promotes the suggestion to current focus
- Auto-advances to next item after completing current focus
- Shows empty state celebration when all items are complete
TDD: 14 tests covering focus, suggestion, empty states, and actions
