refactor: 移除 Study 和 Review 页面大标题

- 页面标题统一由全局页头展示，移除各页面内 h1
- 刷新按钮迁移至操作区/工具栏，功能不变
- Study 页开始任务改为软导航 router.push

src/components/Study.vue

@@ -50,7 +50,7 @@ const startTask = () => {
     ElMessage.warning("请先选择一个任务");
     return;
   }
-  window.location.assign(`/start-task/${encodeURIComponent(selectedTask.value.taskNum)}`);
+  router.push(`/start-task/${encodeURIComponent(selectedTask.value.taskNum)}`);
 };
 
 const addTask = () => {
@@ -86,14 +86,6 @@ onMounted(() => {
 
 <template>
   <section class="study-page">
-    <header class="surface-card page-head">
-      <div>
-        <h1 class="page-title">学习任务</h1>
-        <p class="page-subtitle">先选任务，再确认材料与优先级，然后开始一次学习会话</p>
-      </div>
-      <el-button type="success" plain @click="fetchTasks" :loading="loading">刷新列表</el-button>
-    </header>
-
     <div class="layout-grid">
       <aside class="surface-card task-list">
         <div class="block-title">任务清单</div>
@@ -142,6 +134,7 @@ onMounted(() => {
 
         <aside class="surface-card action-card">
           <div class="block-title">常用操作</div>
+          <el-button size="large" @click="fetchTasks" :loading="loading">刷新列表</el-button>
           <el-button type="success" size="large" @click="startTask">开始任务</el-button>
           <el-button size="large" @click="addTask">添加任务</el-button>
           <el-button size="large" @click="changeTask">更新任务</el-button>
@@ -159,14 +152,6 @@ onMounted(() => {
   gap: 14px;
 }
 
-.page-head {
-  padding: 18px 20px;
-  display: flex;
-  align-items: flex-start;
-  justify-content: space-between;
-  gap: 12px;
-}
-
 .layout-grid {
   display: grid;
   grid-template-columns: 280px 1fr;