feat(web): align attachment display with DeepSeek Chat via ui-attachment atoms

Single-click original preview in the composer rail and chat history; remove
control inside the thumbnail, revealed on hover/focus (always on touch);
hidden-scrollbar rail overflow paged by edge arrows with wheel panning and
end-reveal on add; image-intake rejections and prompt failures announce as a
transient top-center toast instead of inline strips.

The attachment atoms move to a new zero-cordis package
@deepseek-ai/dsh-client-ui-attachment (rail, message gallery, lightbox),
seeded as a platform module; the toast is a ui-primitives atom. Strings
arrive as label props bridged from the conversation dictionary.

packages/client/README.zh.md

@@ -18,6 +18,7 @@ dsh web GUI 的浏览器侧：shell 启动、浏览器与宿主通信、共享 U
 | [`ui-slots/`](ui-slots/README.md) | 定义 UI 功能注册和组合扩展 slot 的方式。 |
 | [`ui-theme/`](ui-theme/README.md) | 应用所选颜色主题。 |
 | [`ui-primitives/`](ui-primitives/README.md) | 提供共享 React 控件、图标和内容渲染器。 |
+| [`ui-attachment/`](ui-attachment/README.md) | 提供附件展示原子组件：草稿图片栏、消息画廊与灯箱。 |
 | [`ui-layout/`](ui-layout/README.md) | 排列应用的主要区域。 |
 | [`ui-sidebar/`](ui-sidebar/README.md) | 展示 Workspace 与会话导航。 |
 | [`ui-workspace/`](ui-workspace/README.md) | 提供 Workspace 选择与创建界面。 |