feat: 故事加载页接收并传递封面 URL

- 新增 storyCoverUrl 变量，从 SSE done 事件中读取 cover_url
- 新增 cover SSE 阶段的进度展示（正在绘制故事封面...）
- 将 cover_url 传递给 StoryDetailPage 和返回结果

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

airhub_app/lib/pages/story_loading_page.dart

@@ -72,6 +72,7 @@ class _StoryLoadingPageState extends State<StoryLoadingPage> {
       String buffer = '';
       String? storyTitle;
       String? storyContent;
+      String storyCoverUrl = '';
 
       await for (final chunk in response.stream.transform(utf8.decoder)) {
         buffer += chunk;
@@ -109,9 +110,13 @@ class _StoryLoadingPageState extends State<StoryLoadingPage> {
               case 'parsing':
                 _updateProgress(progress / 100, '正在编制最后的魔法...');
                 break;
+              case 'cover':
+                _updateProgress(progress / 100, '正在绘制故事封面...');
+                break;
               case 'done':
                 storyTitle = event['title'] as String? ?? '卡皮巴拉的故事';
                 storyContent = event['content'] as String? ?? '';
+                storyCoverUrl = event['cover_url'] as String? ?? '';
                 _updateProgress(1.0, '大功告成！');
                 break;
               case 'error':
@@ -142,6 +147,7 @@ class _StoryLoadingPageState extends State<StoryLoadingPage> {
               story: {
                 'title': storyTitle,
                 'content': storyContent,
+                'cover_url': storyCoverUrl,
               },
             ),
           ),
@@ -154,6 +160,7 @@ class _StoryLoadingPageState extends State<StoryLoadingPage> {
               'action': 'saved',
               'title': storyTitle,
               'content': storyContent,
+              'cover_url': storyCoverUrl,
             });
           } else {
             Navigator.of(context).pop(result);