feat(app): 编辑器增强 — 查看模式 + 图层排序 + 标签/贴纸动态化

- EditorPage 新增查看模式: 打开已保存日记默认只读，编辑按钮切换
- EditorBloc 新增 ElementLayerChanged 事件，支持置顶/置底图层排序
- DraggableElement 添加图层控制按钮 (置顶/置底/删除)
- TagPanel 标签建议改为从日记历史动态生成 (Top 10 频率)
- StickerPickerSheet 重构，预留 API 扩展点

app/lib/features/editor/bloc/editor_bloc.dart

@@ -99,6 +99,16 @@ class ElementSelected extends EditorEvent {
   ElementSelected(this.elementId);
 }
 
+/// 图层顺序调整方向
+enum LayerChange { bringToFront, sendToBack }
+
+/// 调整元素图层顺序
+class ElementLayerChanged extends EditorEvent {
+  final String elementId;
+  final LayerChange change;
+  ElementLayerChanged({required this.elementId, required this.change});
+}
+
 // --- 工具栏事件 ---
 
 /// 切换活动工具
@@ -335,6 +345,7 @@ class EditorBloc extends Bloc<EditorEvent, EditorState> {
     on<ElementResized>(_onElementResized);
     on<ElementRotated>(_onElementRotated);
     on<ElementSelected>(_onElementSelected);
+    on<ElementLayerChanged>(_onElementLayerChanged);
     on<ElementsLoaded>(_onElementsLoaded);
 
     // 日记加载事件
@@ -492,6 +503,36 @@ class EditorBloc extends Bloc<EditorEvent, EditorState> {
     ));
   }
 
+  /// 调整元素图层顺序 — 置顶或置底
+  void _onElementLayerChanged(
+    ElementLayerChanged event,
+    Emitter<EditorState> emit,
+  ) {
+    final elements = List<JournalElement>.from(state.elements);
+    final index = elements.indexWhere((e) => e.id == event.elementId);
+    if (index == -1) return;
+
+    switch (event.change) {
+      case LayerChange.bringToFront:
+        // 设为最大 zIndex + 1
+        final maxZ = elements.fold<int>(
+          0,
+          (max, e) => e.zIndex > max ? e.zIndex : max,
+        );
+        elements[index] = elements[index].copyWith(zIndex: maxZ + 1);
+      case LayerChange.sendToBack:
+        // 设为最小 zIndex - 1
+        final minZ = elements.fold<int>(
+          0,
+          (min, e) => e.zIndex < min ? e.zIndex : min,
+        );
+        elements[index] = elements[index].copyWith(zIndex: minZ - 1);
+    }
+
+    emit(state.copyWith(elements: elements, isDirty: true));
+    _scheduleAutoSave();
+  }
+
   void _onElementsLoaded(ElementsLoaded event, Emitter<EditorState> emit) {
     emit(state.copyWith(elements: event.elements));
   }