feat(tasks): add read-only sharing permission for task lists

Task list shares now carry a permission (editor/viewer). The server rejects
writes from viewers with 403, and the UI hides or disables edit controls
for read-only lists.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/storage/sqlite_test.go

@@ -1,6 +1,7 @@
 package storage
 
 import (
+	"errors"
 	"path/filepath"
 	"testing"
 
@@ -118,7 +119,7 @@ func TestStoreLifecycle(t *testing.T) {
 	}
 
 	// Test TaskList Sharing with Team
-	shareTeam, err := store.AddTaskListShare(taskList.ID, "team", team.ID, user.ID)
+	shareTeam, err := store.AddTaskListShare(taskList.ID, "team", team.ID, "editor", user.ID)
 	if err != nil || shareTeam.ID == "" {
 		t.Fatalf("AddTaskListShare team failed: %v", err)
 	}
@@ -147,6 +148,45 @@ func TestStoreLifecycle(t *testing.T) {
 	if err != nil {
 		t.Fatalf("CreateUser collab failed: %v", err)
 	}
+
+	// Test read-only TaskList share
+	ownerTask, err := store.CreateThoughtWithDetails(user.ID, "Rasen mähen", "normal", "manual", "", taskList.ID, "todo", "")
+	if err != nil {
+		t.Fatalf("CreateThoughtWithDetails in own list failed: %v", err)
+	}
+	if _, err := store.UpdateThoughtTriage(user.ID, ownerTask.ID, "keep", "task", "", "", taskList.ID); err != nil {
+		t.Fatalf("UpdateThoughtTriage in own list failed: %v", err)
+	}
+	if _, err := store.AddTaskListShare(taskList.ID, "user", user2.ID, "viewer", user.ID); err != nil {
+		t.Fatalf("AddTaskListShare viewer failed: %v", err)
+	}
+	if perm := store.GetTaskListPermission(user2.ID, taskList.ID); perm != "viewer" {
+		t.Fatalf("expected viewer permission, got %q", perm)
+	}
+	viewerTasks, err := store.GetTasks(user2.ID, taskList.ID)
+	if err != nil || len(viewerTasks) == 0 {
+		t.Fatalf("viewer should see tasks of shared list: %v", err)
+	}
+	done := true
+	if _, err := store.UpdateThoughtDetails(user2.ID, ownerTask.ID, nil, &done, nil, nil, nil, nil, nil, nil, nil); !errors.Is(err, ErrTaskListReadOnly) {
+		t.Fatalf("viewer must not update tasks, got %v", err)
+	}
+	if _, err := store.CreateThoughtWithDetails(user2.ID, "Hack", "normal", "manual", "", taskList.ID, "todo", ""); !errors.Is(err, ErrTaskListReadOnly) {
+		t.Fatalf("viewer must not create tasks, got %v", err)
+	}
+	if err := store.DeleteThought(user2.ID, ownerTask.ID); !errors.Is(err, ErrTaskListReadOnly) {
+		t.Fatalf("viewer must not delete tasks, got %v", err)
+	}
+	newTitle := "Umbenannt"
+	if _, err := store.UpdateTaskList(user2.ID, taskList.ID, &newTitle, nil, nil, nil, nil, nil); !errors.Is(err, ErrTaskListReadOnly) {
+		t.Fatalf("viewer must not update task list, got %v", err)
+	}
+	if _, err := store.AddTaskListShare(taskList.ID, "user", user2.ID, "editor", user.ID); err != nil {
+		t.Fatalf("upgrading share to editor failed: %v", err)
+	}
+	if !store.CanEditTaskList(user2.ID, taskList.ID) {
+		t.Fatalf("editor share should allow editing")
+	}
 	newDoc, err := store.CreateDocument(user.ID, "", "Test Note", "📄", "", []domain.DocumentBlock{
 		{ID: "blk_1", Type: "bullet", Content: "Geheim", Level: 0},
 	}, []string{"test"}, false)