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.md

@@ -18,6 +18,7 @@ The browser side of the dsh web GUI: shell boot, browser-host communication, sha
 | [`ui-slots/`](ui-slots/README.md) | Defines how UI features register and compose extension slots. |
 | [`ui-theme/`](ui-theme/README.md) | Applies the selected color theme. |
 | [`ui-primitives/`](ui-primitives/README.md) | Provides shared React controls, icons, and content renderers. |
+| [`ui-attachment/`](ui-attachment/README.md) | Provides attachment display atoms: draft-image rail, message gallery, and lightbox. |
 | [`ui-layout/`](ui-layout/README.md) | Arranges the main application regions. |
 | [`ui-sidebar/`](ui-sidebar/README.md) | Presents workspace and session navigation. |
 | [`ui-workspace/`](ui-workspace/README.md) | Provides workspace selection and creation surfaces. |