Allow task completion even if signatories are still empty, put an indicator for completed tasks with incomplete details.

lib/screens/tasks/task_detail_screen.dart

@@ -426,6 +426,28 @@ class _TaskDetailScreenState extends ConsumerState<TaskDetailScreen>
                 const SizedBox(height: 12),
                 Text(description),
               ],
+
+              // warning banner for completed tasks with missing metadata
+              if (task.status == 'completed' && task.hasIncompleteDetails) ...[
+                const SizedBox(height: 12),
+                Row(
+                  children: [
+                    const Icon(
+                      Icons.warning_amber_rounded,
+                      color: Colors.orange,
+                    ),
+                    const SizedBox(width: 6),
+                    Expanded(
+                      child: Text(
+                        'Task completed but some details are still empty.',
+                        style: Theme.of(context).textTheme.bodyMedium?.copyWith(
+                          color: Theme.of(context).colorScheme.onSurfaceVariant,
+                        ),
+                      ),
+                    ),
+                  ],
+                ),
+              ],
               const SizedBox(height: 16),
               // Collapsible tabbed details: Assignees / Type & Category / Signatories
               ExpansionTile(