feat(app): 编辑器未保存确认 + 日历今天按钮

D.2.3 编辑器未保存确认:
- _handleBack 检查 state.isDirty，有未保存修改时弹出确认对话框
- 对话框: '放弃编辑？' / '你有未保存的修改' / [继续编辑] [放弃]

D.2.4 日历今天按钮:
- _MonthNavigator 新增 onToday 回调
- 不在当前月时显示 '今天' 文字按钮，点击跳回当月
- _isCurrentMonth 辅助判断

app/lib/features/calendar/views/calendar_page.dart

@@ -83,6 +83,9 @@ class _CalendarView extends StatelessWidget {
                 );
                 context.read<CalendarBloc>().add(CalendarMonthChanged(next));
               },
+              onToday: () {
+                context.read<CalendarBloc>().add(CalendarMonthChanged(DateTime.now()));
+              },
             ),
 
             // 视图模式切换
@@ -543,11 +546,13 @@ class _MonthNavigator extends StatelessWidget {
     required this.month,
     required this.onPrevious,
     required this.onNext,
+    this.onToday,
   });
 
   final DateTime month;
   final VoidCallback onPrevious;
   final VoidCallback onNext;
+  final VoidCallback? onToday;
 
   @override
   Widget build(BuildContext context) {
@@ -580,6 +585,22 @@ class _MonthNavigator extends StatelessWidget {
               fontWeight: FontWeight.bold,
             ),
           ),
+          // "今天" 按钮 — 不在当前月时显示
+          if (onToday != null && !_isCurrentMonth(month))
+            Padding(
+              padding: const EdgeInsets.only(left: 8),
+              child: TextButton(
+                onPressed: onToday,
+                style: TextButton.styleFrom(
+                  padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 4),
+                  minimumSize: Size.zero,
+                  tapTargetSize: MaterialTapTargetSize.shrinkWrap,
+                ),
+                child: const Text('今天', style: TextStyle(fontSize: 13)),
+              ),
+            )
+          else
+            const SizedBox(width: 8),
           SizedBox(
             width: 44,
             height: 44,
@@ -607,6 +628,11 @@ class _MonthNavigator extends StatelessWidget {
     ];
     return '${date.year}年 ${months[date.month - 1]}';
   }
+
+  bool _isCurrentMonth(DateTime date) {
+    final now = DateTime.now();
+    return date.year == now.year && date.month == now.month;
+  }
 }
 
 // ===== 星期标题行 =====