feat: 添加路由标题元数据，固定页头为 sticky

- 为所有子页面路由添加 meta.title 供页头动态展示
- 页头改为 position: sticky，滚动时固定在顶部

src/App.vue

@@ -42,6 +42,10 @@ const route = useRoute();
 .shell-header {
   height: auto;
   padding: 18px 20px 0;
+  position: sticky;
+  top: 0;
+  z-index: 10;
+  background: var(--surface);
 }
 
 .shell-main {

src/router/index.ts

@@ -20,23 +20,23 @@ const routes: RouteRecordRaw[] = [
     path: "/study",
     name: "study",
     component: () => import("@/components/Study.vue"),
-    meta: { requiresAuth: true },
+    meta: { requiresAuth: true, title: "学习任务" },
   },
   {
     path: "/review",
     component: () => import("@/components/Review.vue"),
-    meta: { requiresAuth: true },
+    meta: { requiresAuth: true, title: "复习总览" },
   },
   {
     path: "/review/detail/:type/:id",
     component: () => import("@/components/ReviewDetail.vue"),
-    meta: { requiresAuth: true },
+    meta: { requiresAuth: true, title: "复习详情" },
   },
   {
     path: "/start-task/:taskNum",
     name: "start-task",
     component: StartTask,
-    meta: { requiresAuth: true },
+    meta: { requiresAuth: true, title: "学习会话" },
   },
   {
     path: "/add-task",
@@ -44,6 +44,7 @@ const routes: RouteRecordRaw[] = [
     component: () => import("@/components/TaskForm.vue"),
     meta: {
       requiresAuth: true,
+      title: "创建学习任务",
       action: "add",
       buttonText: "添加",
     },
@@ -54,6 +55,7 @@ const routes: RouteRecordRaw[] = [
     component: () => import("@/components/TaskForm.vue"),
     meta: {
       requiresAuth: true,
+      title: "更新学习任务",
       action: "update",
       buttonText: "更新",
     },