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/domain/models.go

@@ -181,6 +181,7 @@ type Thought struct {
 	KanbanStatus   string        `json:"kanbanStatus,omitempty"` // todo | in_progress | done
 	Notes          string        `json:"notes,omitempty"`
 	Subtasks       []SubTask     `json:"subtasks"`
+	Assignees      []string      `json:"assignees"` // user IDs; only people with access to the task list
 	AlarmSent      bool          `json:"alarmSent,omitempty"`
 	SortOrder      int           `json:"sortOrder"`
 	CreatedAt      string        `json:"createdAt"`
@@ -426,24 +427,27 @@ type ListItem struct {
 	IsCompleted     bool   `json:"isCompleted"`
 	CompletedBy     string `json:"completedBy,omitempty"`
 	CompletedByName string `json:"completedByName,omitempty"`
+	CreatedBy       string `json:"createdBy,omitempty"`
+	CreatedByName   string `json:"createdByName,omitempty"`
 	SortOrder       int    `json:"sortOrder"`
 	CreatedAt       string `json:"createdAt"`
 	CompletedAt     string `json:"completedAt,omitempty"`
 }
 
 type SharedList struct {
-	ID        string      `json:"id"`
-	UserID    string      `json:"userId"`
-	OwnerName string      `json:"ownerName,omitempty"`
-	IsOwner   bool        `json:"isOwner"`
-	Role      string      `json:"role,omitempty"` // editor | viewer
-	Title     string      `json:"title"`
-	Icon      string      `json:"icon"`
-	ListType  string      `json:"listType"` // shopping | checklist | vacation
-	Items     []ListItem  `json:"items"`
-	Shares    []ListShare `json:"shares,omitempty"`
-	UpdatedAt string      `json:"updatedAt"`
-	CreatedAt string      `json:"createdAt"`
+	ID        string           `json:"id"`
+	UserID    string           `json:"userId"`
+	OwnerName string           `json:"ownerName,omitempty"`
+	IsOwner   bool             `json:"isOwner"`
+	Role      string           `json:"role,omitempty"` // editor | viewer
+	Title     string           `json:"title"`
+	Icon      string           `json:"icon"`
+	ListType  string           `json:"listType"` // shopping | checklist | vacation
+	Items     []ListItem       `json:"items"`
+	Shares    []ListShare      `json:"shares,omitempty"`
+	Members   []TaskListMember `json:"members,omitempty"` // owner + everyone the list is shared with
+	UpdatedAt string           `json:"updatedAt"`
+	CreatedAt string           `json:"createdAt"`
 }
 
 type APIKey struct {