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/ui-attachment/src/index.ts

@@ -0,0 +1,14 @@
+/**
+ * Pure React attachment atoms (zero cordis): the composer draft-image rail,
+ * the chat-history image gallery, and the original-image lightbox. Owners
+ * resolve every string through their own locale namespace and pass it down;
+ * nothing here reads application state.
+ * @module @deepseek-ai/dsh-client-ui-attachment
+ */
+
+export { AttachmentRail } from './AttachmentRail.tsx'
+export type { AttachmentRailItem, AttachmentRailLabels } from './AttachmentRail.tsx'
+export { ImageLightbox } from './ImageLightbox.tsx'
+export type { ImageLightboxLabels } from './ImageLightbox.tsx'
+export { ImageGallery, MessageImage } from './MessageImage.tsx'
+export type { ImageLoader, MessageImageLabels } from './MessageImage.tsx'