- Replace POST /invitation/:id/accept|decline with PUT /invitation/:id
- Add DELETE /invitation/:id (full CRUD: GET, POST, PUT, DELETE)
- Validate invitation recipient exists before creating invitation (404)
- Block self-invitation (400)
- Propagate server error messages to inline card feedback
- Add Loeschen button to invitation cards (demonstrates DELETE in UI)
- Improve empty state with icon and descriptive text
