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/README.md

@@ -18,6 +18,7 @@ Referenz für Entwickler und Administratoren. Alle Angaben wurden gegen den Code
 | [sharing-and-permissions.md](sharing-and-permissions.md) | Rollen, Gruppen, Freigaben |
 | [reverse-proxy.md](reverse-proxy.md) | Nginx, Traefik, Caddy, Cloudflare |
 | [theming.md](theming.md) | Themes und Design-Variablen |
+| [ui-patterns.md](ui-patterns.md) | Einheitliche Bedienelemente: Personensuche, Profilbild-Stapel, Zähler |
 | [flaws.md](flaws.md) | Bekannte Schwachstellen und technische Schulden |
 
 ## Überblick in 60 Sekunden