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}`, {

frontend/src/lib/types.ts

@@ -62,6 +62,7 @@ export interface Thought {
   kanbanStatus?: string;
   notes?: string;
   subtasks?: SubTask[];
+  assignees?: string[]; // user IDs, only people with access to the task list
   alarmSent?: boolean;
   sortOrder?: number;
   createdAt: string;
@@ -349,6 +350,8 @@ export interface ListItem {
   isCompleted: boolean;
   completedBy?: string;
   completedByName?: string;
+  createdBy?: string;
+  createdByName?: string;
   sortOrder: number;
   createdAt: string;
   completedAt?: string;
@@ -378,6 +381,7 @@ export interface SharedList {
   listType: 'shopping' | 'checklist' | 'vacation';
   items: ListItem[];
   shares?: ListShare[];
+  members?: TaskListMember[]; // owner + everyone the list is shared with
   updatedAt: string;
   createdAt: string;
 }