feat(tasks): assign tasks in shared lists, unified people search, list avatars

- Tasks in shared task lists can be assigned to one or more people with access
  (task_assignees); assignments are pruned whenever access is lost
- UserAutocomplete: type-to-search everywhere, nothing before typing, max 3 results,
  first match preselected
- Task list and list tabs: avatar stack of members and round monochrome counter
- Shared list items remember who added them (list_items.created_by)
- Doku/ui-patterns.md documents the shared UI patterns

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/seed.go

@@ -119,7 +119,7 @@ func (s *Store) SeedStarterContent(user *domain.User) error {
 			{"Spülmaschinentabs", "Drogerie", "1 Pack"},
 		}
 		for i, it := range items {
-			item, err := s.CreateListItem(shop.ID, it.content, it.category, it.qty, i)
+			item, err := s.CreateListItem(shop.ID, it.content, it.category, it.qty, i, user.ID)
 			if err == nil && (i == 1 || i == 6 || i == 8) {
 				_, _ = s.ToggleListItem(item.ID, true, user.ID, user.DisplayName)
 			}
@@ -132,7 +132,7 @@ func (s *Store) SeedStarterContent(user *domain.User) error {
 			{"Erste-Hilfe-Set", "Ausrüstung"}, {"Stirnlampe", "Ausrüstung"}, {"Trinkflaschen", "Ausrüstung"},
 			{"Personalausweise", "Dokumente"}, {"Hüttenschlafsack", "Ausrüstung"},
 		} {
-			_, _ = s.CreateListItem(packing.ID, it.content, it.category, "", i)
+			_, _ = s.CreateListItem(packing.ID, it.content, it.category, "", i, user.ID)
 		}
 	}