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/sqlite_test.go

@@ -54,7 +54,7 @@ func TestStoreLifecycle(t *testing.T) {
 	if err != nil {
 		t.Fatalf("CreateList failed: %v", err)
 	}
-	item, err := store.CreateListItem(lst.ID, "Hafermilch", "Kühlregal", "2L", 0)
+	item, err := store.CreateListItem(lst.ID, "Hafermilch", "Kühlregal", "2L", 0, user.ID)
 	if err != nil {
 		t.Fatalf("CreateListItem failed: %v", err)
 	}