feat(task): record and display task completion time

Add DoneAt to the download task model: set when a task finishes and
cleared when it is downloaded again. The value flows through the API
model and task record, and is shown as "Completed at" in the task
drawer info tab next to the creation time.

ui/flutter/test/features/tasks/domain/task_record_test.dart

@@ -4,7 +4,7 @@ import 'package:gopeed/core/icons/gopeed_icons.dart';
 import 'package:gopeed/features/tasks/domain/task_record.dart';
 
 void main() {
-  api.Task buildTask(List<int> selection, int size) => api.Task.fromJson({
+  api.Task buildTask(List<int> selection, int size, {String? doneAt}) => api.Task.fromJson({
     'id': 'bt-selection',
     'name': 'bundle',
     'protocol': 'bt',
@@ -12,6 +12,7 @@ void main() {
     'uploading': false,
     'createdAt': '2026-01-01T00:00:00Z',
     'updatedAt': '2026-01-01T00:00:00Z',
+    'doneAt': doneAt,
     'meta': {
       'req': {'url': 'magnet:?xt=urn:btih:test'},
       'opts': {'path': '/downloads', 'selectFiles': selection},
@@ -43,6 +44,13 @@ void main() {
     expect(record.totalBytes, 60);
   });
 
+  test('completion time maps from api and stays null for legacy tasks', () {
+    final completed = TaskRecord.fromApi(buildTask([], 60, doneAt: '2026-01-02T03:04:05Z'));
+    expect(completed.doneAt, DateTime.parse('2026-01-02T03:04:05Z'));
+    final legacy = TaskRecord.fromApi(buildTask([], 60));
+    expect(legacy.doneAt, isNull);
+  });
+
   test('file names preserve hash and question mark characters for type detection', () {
     expect(
       taskFileTypeIcon('Slow English Podcast #2 _ Eating, Talking, and Learning Natural English.mp4'),