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>

Doku/api/rest.md

@@ -38,7 +38,7 @@ Basis: `/api/v1`. Antworten sind JSON; Fehler haben die Form `{"error": "Text"}`
 |---|---|---|
 | POST | `/inbox` | Schnellerfassung. Body: `content`, `priority`, `source`, `dueDate`, `target` (`inbox`/`task`/`list_item`/`document`), `targetListId`, `taskListId` (snake_case-Varianten werden akzeptiert) |
 | GET | `/thoughts?status=` | Gedanken (`inbox`, `kept`, `converted`, `discarded`, `all`) |
-| GET/PATCH/DELETE | `/thoughts/{id}` | Einzelabruf; Teiländerung (`content`, `isCompleted`, `dueDate`, `priority`, `taskListId`, `kanbanStatus`, `notes`, `subtasks`, `tags`); Löschen |
+| GET/PATCH/DELETE | `/thoughts/{id}` | Einzelabruf; Teiländerung (`content`, `isCompleted`, `dueDate`, `priority`, `taskListId`, `kanbanStatus`, `notes`, `subtasks`, `tags`, `assignees`); Löschen. `assignees` ersetzt die Zuweisungen; nur Personen mit Zugriff auf die Liste, sonst `403` |
 | DELETE | `/thoughts/discarded/purge` | Papierkorb leeren |
 | GET | `/triage/deck` | Posteingang (max. 100) |
 | PATCH | `/triage/{id}` | `action`: `keep`/`discard`/`restore`; mit `convertTo`: `task`/`document`/`list_item` |