feat: 页头动态标题与返回首页按钮

- 子页面显示对应路由标题，首页/登录页保留品牌信息
- 非首页非登录页显示"← 返回首页"按钮

src/components/MyHead.vue

@@ -1,6 +1,17 @@
 <template>
   <div class="head surface-card">
-    <div class="brand-block">
+    <el-button
+      v-if="showBackButton"
+      text
+      class="back-btn"
+      @click="router.push('/welcome')"
+    >
+      ← 返回首页
+    </el-button>
+    <template v-if="pageTitle">
+      <h1 class="head-title">{{ pageTitle }}</h1>
+    </template>
+    <div v-else class="brand-block">
       <p class="brand-title">学习进度跟踪系统</p>
       <p class="brand-subtitle">把学习变成可记录、可回顾、可持续的日常流程</p>
     </div>
@@ -25,6 +36,8 @@ import { logout } from "@/api/login";
 
 const route = useRoute();
 const isLoginRoute = computed(() => route.path === "/login");
+const showBackButton = computed(() => route.path !== "/login" && route.path !== "/welcome");
+const pageTitle = computed(() => route.meta?.title as string | undefined);
 
 const handleLogout = async () => {
   try {
@@ -63,6 +76,24 @@ const handleLogout = async () => {
   color: var(--text-secondary);
 }
 
+.head-title {
+  margin: 0;
+  font-size: clamp(18px, 2.4vw, 24px);
+  font-weight: 700;
+  color: var(--green-900);
+  flex: 1;
+}
+
+.back-btn {
+  font-size: 14px;
+  color: var(--green-700);
+  padding: 0 4px;
+  flex-shrink: 0;
+}
+.back-btn:hover {
+  color: var(--green-900);
+}
+
 .logout-btn {
   border-color: var(--green-600);
   color: var(--green-700);