fix(app): 日记保存逻辑修复 — EditorPage 改为 StatefulWidget + 更新合并编辑器状态

根因分析：
1. EditorPage 是 StatelessWidget，savedJournalId 作为 build() 局部变量
   每次重建都重置为 null，导致每次自动保存都走新建而非更新分支
2. 更新分支直接 repo.updateJournal(existing)，没有把编辑器当前
   状态（标题/心情/标签）合并到已有日记中

修复：
- EditorPage 改为 StatefulWidget，_savedJournalId 存储在 State 中
- 更新分支用 existing.copyWith() 合并编辑器当前状态后保存

app/lib/features/editor/views/editor_page.dart

@@ -37,12 +37,26 @@ import '../widgets/brush_panel.dart';
 import '../widgets/dot_grid_painter.dart';
 
 /// 手账编辑器页面
-class EditorPage extends StatelessWidget {
+class EditorPage extends StatefulWidget {
   final String? journalId;
   final String? templateId;
 
   const EditorPage({super.key, this.journalId, this.templateId});
 
+  @override
+  State<EditorPage> createState() => _EditorPageState();
+}
+
+class _EditorPageState extends State<EditorPage> {
+  /// 跟踪已保存的日记 ID — 新建日记首次保存后赋值
+  String? _savedJournalId;
+
+  @override
+  void initState() {
+    super.initState();
+    _savedJournalId = widget.journalId;
+  }
+
   @override
   Widget build(BuildContext context) {
     // 从 Provider 树获取 JournalRepository（IsarJournalRepository）
@@ -50,10 +64,6 @@ class EditorPage extends StatelessWidget {
     // 从 Provider 树获取 SyncEngine（同步到后端）
     final syncEngine = context.read<SyncEngine>();
 
-    // 可变闭包变量：跟踪已保存的日记 ID
-    // 新建日记首次保存后赋值，后续自动更新使用此 ID
-    String? savedJournalId = journalId;
-
     return BlocProvider(
       create: (_) => EditorBloc(
         onSave: (state) async {
@@ -66,7 +76,7 @@ class EditorPage extends StatelessWidget {
             }
 
             await _persistState(
-              repo, state, (id) => savedJournalId = id, savedJournalId,
+              repo, state, (id) => _savedJournalId = id, _savedJournalId,
               syncEngine: syncEngine,
               authorId: authorId,
             );
@@ -76,12 +86,12 @@ class EditorPage extends StatelessWidget {
         },
       ),
       child: _EditorView(
-        journalId: journalId,
-        templateId: templateId,
-        savedJournalId: savedJournalId,
+        journalId: widget.journalId,
+        templateId: widget.templateId,
+        savedJournalId: _savedJournalId,
         repo: repo,
         onSaveComplete: () {
-          _showShareSheetAndNavigate(context, repo, savedJournalId);
+          _showShareSheetAndNavigate(context, repo, _savedJournalId);
         },
       ),
     );
@@ -137,7 +147,14 @@ class EditorPage extends StatelessWidget {
       // --- 更新已有日记 ---
       final existing = await repo.getJournal(savedJournalId);
       if (existing != null) {
-        await repo.updateJournal(existing);
+        // 将编辑器当前状态合并到已有日记中
+        final updated = existing.copyWith(
+          title: state.title.isNotEmpty ? state.title : existing.title,
+          mood: state.selectedMood,
+          tags: state.tags.isNotEmpty ? state.tags : existing.tags,
+          updatedAt: now,
+        );
+        await repo.updateJournal(updated);
 
         // 入队 SyncEngine 等待同步到后端
         syncEngine.enqueue(PendingOperation(