优化登录任务界面 + 修复时间显示 + 增加删除功能

- 筛选栏改为单行横排，概览精简为一行文字
- 任务列表全宽展示，日志移到底部可折叠
- 页面使用百分比布局，一屏展示无外层滚动
- 登录任务增加单条删除和批量删除功能
- 后端: datetime.utcnow 替换为 datetime.now(timezone.utc)
- 后端: schemas 序列化统一输出带时区 ISO 格式
- 前端: 新增 utils/time.ts 统一时间格式化工具
- 前端: CookiePage/LoginTasksPage 时间列使用 formatTime()

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/backend/routers/login.py

@@ -111,6 +111,38 @@ def list_tasks(
     return result
 
 
+@router.delete("/tasks/{task_id}")
+def delete_task(
+    task_id: int,
+    db: Session = Depends(get_db),
+    current: User = Depends(require_permission("login:batch")),
+):
+    """删除单个登录任务。"""
+    task = db.query(LoginTask).filter(LoginTask.id == task_id).first()
+    if not task:
+        raise HTTPException(status_code=404, detail="任务不存在")
+    db.delete(task)
+    db.commit()
+    return {"message": "已删除", "success": True}
+
+
+@router.delete("/tasks")
+def delete_tasks(
+    task_ids: str = "",
+    db: Session = Depends(get_db),
+    current: User = Depends(require_permission("login:batch")),
+):
+    """批量删除登录任务。"""
+    if not task_ids:
+        raise HTTPException(status_code=400, detail="请指定任务ID")
+    ids = [int(x) for x in task_ids.split(",") if x.strip().isdigit()]
+    if not ids:
+        raise HTTPException(status_code=400, detail="无效的任务ID")
+    deleted = db.query(LoginTask).filter(LoginTask.id.in_(ids)).delete(synchronize_session=False)
+    db.commit()
+    return {"message": f"已删除 {deleted} 个任务", "deleted": deleted, "success": True}
+
+
 @router.post("/stop/{batch_id}")
 def stop_batch(
     batch_id: str,