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>

frontend/src/lib/api.ts

@@ -362,6 +362,7 @@ export const api = {
       notes?: string;
       subtasks?: SubTask[];
       tags?: string[];
+      assignees?: string[];
     }
   ): Promise<Thought> {
     return request<Thought>(`/api/v1/thoughts/${thoughtId}`, {