feat(agent): 支持自建风格与智能体双模式对话系统

- 新增 `source` 字段区分智能体(`agent`)与自建风格(`prompt`)两种对话来源
- 前端统一对话组件，根据来源动态构建请求参数、显示不同样式与文案
- 后端重构 Dify 会话与消息获取逻辑，支持合并查询 Pro 与 Standard 两个 Dify App 的会话历史
- 实现复合游标分页机制，支持跨双数据源的高效分页
- 新增 `clipboard-polyfill` 依赖，统一剪贴板复制功能，提升非 HTTPS 环境兼容性
- 扩展历史记录面板，支持按来源加载对应会话与消息
- 调整侧边抽屉宽度，优化大屏显示体验

frontend/app/web-gold/src/components/ui/sheet/SheetContent.vue

@@ -39,9 +39,9 @@ const forwarded = useForwardPropsEmits(delegatedProps, emits)
       :class="cn(
         'bg-background data-[state=open]:animate-in data-[state=closed]:animate-out fixed z-50 flex flex-col gap-4 shadow-lg transition ease-in-out data-[state=closed]:duration-300 data-[state=open]:duration-500',
         side === 'right'
-          && 'data-[state=closed]:slide-out-to-right data-[state=open]:slide-in-from-right inset-y-0 right-0 h-full w-3/4 border-l sm:max-w-sm',
+          && 'data-[state=closed]:slide-out-to-right data-[state=open]:slide-in-from-right inset-y-0 right-0 h-full w-3/4 border-l sm:max-w-2xl',
         side === 'left'
-          && 'data-[state=closed]:slide-out-to-left data-[state=open]:slide-in-from-left inset-y-0 left-0 h-full w-3/4 border-r sm:max-w-sm',
+          && 'data-[state=closed]:slide-out-to-left data-[state=open]:slide-in-from-left inset-y-0 left-0 h-full w-3/4 border-r sm:max-w-2xl',
         side === 'top'
           && 'data-[state=closed]:slide-out-to-top data-[state=open]:slide-in-from-top inset-x-0 top-0 h-auto border-b',
         side === 'bottom'