feat: add quick action buttons (preview/undo/reset) to sidebar

Sidebar now has 快捷操作 section matching Streamlit app functionality:
- 预览 — sends "预览报表" to preview current JRXML
- 撤销 — sends "撤销上一步修改" to revert last change
- 重置 — sends "重新来，清空当前报表" to reset session

Session store now tracks history_states for undo availability check.

frontend/src/App.vue

@@ -114,7 +114,7 @@ async function handleSend(text: string, files: File[]) {
 
 <template>
   <div class="app-layout">
-    <Sidebar />
+    <Sidebar @quickAction="(text) => handleSend(text, [])" />
 
     <main class="main-area">
       <div class="chat-container" ref="chatContainer">