fix: 选题页状态过滤器改用服务端过滤，移除watch覆写URL参数导致无法清除筛选的问题

- fetchTopics 根据 filterStatus 发送 status/today 参数到后端，而非全量拉取后客户端过滤
- 移除 watch.topics 处理器，解决 URL 参数反复覆写用户筛选操作的问题
- 今日新增（?today=true）和服务端 status 参数均走 API 过滤

platform/frontend/topics.html

@@ -235,10 +235,16 @@ const TopicsApp = {
         async fetchTopics() {
             this.loadingTable = true;
             try {
-                const params = this.filterStatus === 'today' ? '?today=true' : '';
-                const data = await this.api('/api/topics' + params);
+                const params = new URLSearchParams();
+                if (this.filterStatus === 'today') {
+                    params.set('today', 'true');
+                } else if (this.filterStatus) {
+                    params.set('status', this.filterStatus);
+                }
+                const qs = params.toString();
+                const data = await this.api('/api/topics' + (qs ? '?' + qs : ''));
                 this.topics = data || [];
-                this.$message.success('选题加载成功');
+                if (qs) this.$message.success(`加载 ${this.filterStatus} 选题成功`);
             } catch (error) {
                 console.error('获取选题失败:', error);
                 this.$message.error(`获取选题失败: ${error.message}`);
@@ -396,12 +402,6 @@ const TopicsApp = {
             .then(r => r.ok ? r.json() : Promise.reject())
             .then(data => { this.currentUser = data.user; this.isAdmin = data.user.role === 'admin'; this.isLoggedIn = true; this.fetchTopics(); this.fetchTodayCount(); })
             .catch(() => { localStorage.removeItem('authToken'); localStorage.removeItem('userRole'); localStorage.removeItem('currentUser'); window.location.href = '/login.html'; });
-    },
-    watch: {
-        topics() {
-            const urlFilter = new URLSearchParams(window.location.search).get('filter');
-            if (urlFilter) this.filterStatus = urlFilter;
-        }
     }
 };
 const app = Vue.createApp(TopicsApp);