Add server selection page

Add a new page for server selection and update backend URL handling.

* **ServerSelection.vue**: Create a new Vue component with a form to input and save the backend server URL to `localStorage`. Add methods to handle form submission and load the saved URL on component mount.
* **index.vue**: Update the `data` method to retrieve the backend URL from `localStorage`. Remove the hardcoded `backurl` value. Add a method to update the backend URL from `localStorage` on component mount. Update API requests to use the dynamic backend URL.
* **router/index.js**: Add a new route for the `ServerSelection` component.

---

For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/SunWuyuan/homeworkpage-frontend?shareId=XXXX-XXXX-XXXX-XXXX).

src/pages/ServerSelection.vue

@@ -0,0 +1,37 @@
+<template>
+  <v-container>
+    <v-form @submit.prevent="saveServerUrl">
+      <v-text-field
+        v-model="serverUrl"
+        label="Backend Server URL"
+        required
+      />
+      <v-btn type="submit" color="primary">Save</v-btn>
+    </v-form>
+  </v-container>
+</template>
+
+<script>
+export default {
+  data() {
+    return {
+      serverUrl: ''
+    };
+  },
+  methods: {
+    saveServerUrl() {
+      localStorage.setItem('backendServerUrl', this.serverUrl);
+      alert('Server URL saved!');
+    },
+    loadServerUrl() {
+      const savedUrl = localStorage.getItem('backendServerUrl');
+      if (savedUrl) {
+        this.serverUrl = savedUrl;
+      }
+    }
+  },
+  mounted() {
+    this.loadServerUrl();
+  }
+};
+</script>

src/pages/index.vue

@@ -133,7 +133,7 @@ export default {
 
   data() {
     return {
-      backurl: import.meta.env.VITE_BACKURL,
+      backurl: localStorage.getItem('backendServerUrl') || '',
       currentEditSubject: null,
       studentList: ["加载中"],
       selectedSet: new Set(),
@@ -162,6 +162,7 @@ export default {
 
   async mounted() {
     try {
+      this.updateBackendUrl();
       await this.initializeData();
     } catch (err) {
       console.error("初始化失败:", err);
@@ -344,6 +345,13 @@ export default {
       this.selectedSet = new Set(res.data.attendance || []);
       this.synced = true;
     },
+
+    updateBackendUrl() {
+      const savedUrl = localStorage.getItem('backendServerUrl');
+      if (savedUrl) {
+        this.backurl = savedUrl;
+      }
+    },
   },
 };
 </script>

src/router/index.js

@@ -34,3 +34,13 @@ router.isReady().then(() => {
 })
 
 export default router
+
+import ServerSelection from '@/pages/ServerSelection.vue'
+
+const routes = [
+  {
+    path: '/server-selection',
+    name: 'ServerSelection',
+    component: ServerSelection,
+  },
+]