feat: 排行榜时间列支持点击排序（created_at，最新在前/最早在前可切换）

internal/web/handlers/speedtest.go

@@ -293,6 +293,8 @@ func (h *Handler) Rankings(c *gin.Context) {
 		col = "upload_mbps"
 	case "latency":
 		col = "latency_ms"
+	case "created":
+		col = "created_at"
 	default:
 		sortField, col = "download", "download_mbps"
 	}

internal/web/templates/index.html

@@ -182,7 +182,7 @@
             <th class="sortable" data-field="latency">延迟 <span class="arrow"></span></th>
             <th class="sortable active" data-field="download">下载 <span class="arrow">↓</span></th>
             <th class="sortable" data-field="upload">上传 <span class="arrow"></span></th>
-            <th style="width:120px">时间</th>
+            <th class="sortable" data-field="created">时间 <span class="arrow"></span></th>
           </tr>
         </thead>
         <tbody id="rankBody"><tr><td colspan="6" class="empty">加载中…</td></tr></tbody>
@@ -528,7 +528,7 @@ document.querySelectorAll('th.sortable').forEach(th => {
       sortOrder = sortOrder === 'desc' ? 'asc' : 'desc'; // 再点切换方向
     } else {
       sortField = field;
-      sortOrder = field === 'latency' ? 'asc' : 'desc'; // 延迟默认升序（低在前），速度默认降序
+      sortOrder = field === 'latency' ? 'asc' : 'desc'; // 延迟默认升序（低在前），其余默认降序（最新/最快在前）
     }
     updateSortHeaders();
     loadRankings();