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/storage/task_assignees_test.go

@@ -0,0 +1,116 @@
+package storage
+
+import (
+	"errors"
+	"path/filepath"
+	"testing"
+)
+
+func TestTaskAssigneesFollowListAccess(t *testing.T) {
+	store, err := NewStore(filepath.Join(t.TempDir(), "assignees.db"))
+	if err != nil {
+		t.Fatalf("NewStore failed: %v", err)
+	}
+	defer store.Close()
+
+	owner, _ := store.EnsureDefaultUser()
+	direct, _ := store.CreateUser("direct@example.com", "hash", "Direkt", "member")
+	viaTeam, _ := store.CreateUser("team@example.com", "hash", "Team", "member")
+	stranger, _ := store.CreateUser("stranger@example.com", "hash", "Fremd", "member")
+
+	tl, err := store.CreateTaskList(owner.ID, "", "", "Haushalt", "🏠", "")
+	if err != nil {
+		t.Fatalf("CreateTaskList failed: %v", err)
+	}
+	task, err := store.CreateThoughtWithDetails(owner.ID, "Fenster putzen", "normal", "manual", "", tl.ID, "todo", "")
+	if err != nil {
+		t.Fatalf("CreateThoughtWithDetails failed: %v", err)
+	}
+	if _, err := store.UpdateThoughtTriage(owner.ID, task.ID, "keep", "task", "", "", tl.ID); err != nil {
+		t.Fatalf("UpdateThoughtTriage failed: %v", err)
+	}
+
+	org, _ := store.CreateOrganization(owner.ID, "Familie")
+	team, err := store.CreateTeam(org.ID, "WG", "")
+	if err != nil {
+		t.Fatalf("CreateTeam failed: %v", err)
+	}
+	_ = store.AddTeamMember(team.ID, owner.ID, "admin")
+	_ = store.AddTeamMember(team.ID, viaTeam.ID, "member")
+	if _, err := store.AddTaskListShare(tl.ID, "user", direct.ID, "editor", owner.ID); err != nil {
+		t.Fatalf("AddTaskListShare user failed: %v", err)
+	}
+	if _, err := store.AddTaskListShare(tl.ID, "team", team.ID, "viewer", owner.ID); err != nil {
+		t.Fatalf("AddTaskListShare team failed: %v", err)
+	}
+
+	if _, err := store.SetTaskAssignees(owner.ID, task.ID, []string{stranger.ID}); !errors.Is(err, ErrAssigneeNoAccess) {
+		t.Fatalf("assigning someone without access must fail, got %v", err)
+	}
+	if _, err := store.SetTaskAssignees(viaTeam.ID, task.ID, []string{viaTeam.ID}); !errors.Is(err, ErrTaskListReadOnly) {
+		t.Fatalf("viewer must not assign, got %v", err)
+	}
+	got, err := store.SetTaskAssignees(direct.ID, task.ID, []string{owner.ID, direct.ID, viaTeam.ID, direct.ID})
+	if err != nil || len(got) != 3 {
+		t.Fatalf("expected 3 assignees, got %v (%v)", got, err)
+	}
+
+	tasks, _ := store.GetTasks(owner.ID, tl.ID)
+	if len(tasks) != 1 || len(tasks[0].Assignees) != 3 {
+		t.Fatalf("GetTasks should carry assignees, got %+v", tasks)
+	}
+
+	// Leaving the team removes the team member's assignment
+	if err := store.RemoveTeamMember(team.ID, viaTeam.ID); err != nil {
+		t.Fatalf("RemoveTeamMember failed: %v", err)
+	}
+	assertAssignees(t, store, owner.ID, task.ID, owner.ID, direct.ID)
+
+	// Removing the direct share removes that user's assignment, the owner stays
+	if err := store.RemoveTaskListShare(tl.ID, direct.ID); err != nil {
+		t.Fatalf("RemoveTaskListShare failed: %v", err)
+	}
+	assertAssignees(t, store, owner.ID, task.ID, owner.ID)
+
+	// Access through a second path keeps the assignment
+	_ = store.AddTeamMember(team.ID, direct.ID, "member")
+	_, _ = store.AddTaskListShare(tl.ID, "user", direct.ID, "editor", owner.ID)
+	if _, err := store.SetTaskAssignees(owner.ID, task.ID, []string{owner.ID, direct.ID}); err != nil {
+		t.Fatalf("re-assign failed: %v", err)
+	}
+	_ = store.RemoveTaskListShare(tl.ID, direct.ID)
+	assertAssignees(t, store, owner.ID, task.ID, owner.ID, direct.ID)
+
+	// Deleting the team takes the last path away
+	if err := store.DeleteTeam(team.ID); err != nil {
+		t.Fatalf("DeleteTeam failed: %v", err)
+	}
+	assertAssignees(t, store, owner.ID, task.ID, owner.ID)
+
+	// Moving the task out of the list drops all assignments
+	empty := ""
+	if _, err := store.UpdateThoughtDetails(owner.ID, task.ID, nil, nil, nil, nil, &empty, nil, nil, nil, nil); err != nil {
+		t.Fatalf("UpdateThoughtDetails failed: %v", err)
+	}
+	assertAssignees(t, store, owner.ID, task.ID)
+}
+
+func assertAssignees(t *testing.T, store *Store, viewerID, taskID string, want ...string) {
+	t.Helper()
+	th, err := store.GetThoughtByID(viewerID, taskID)
+	if err != nil {
+		t.Fatalf("GetThoughtByID failed: %v", err)
+	}
+	if len(th.Assignees) != len(want) {
+		t.Fatalf("expected assignees %v, got %v", want, th.Assignees)
+	}
+	have := map[string]bool{}
+	for _, id := range th.Assignees {
+		have[id] = true
+	}
+	for _, id := range want {
+		if !have[id] {
+			t.Fatalf("expected assignees %v, got %v", want, th.Assignees)
+		}
+	}
+}