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.

tsconfig.base.json

@@ -154,6 +154,7 @@
       "@deepseek-ai/dsh-host-apiproxy/*": ["./packages/host/apiproxy/src/*"],
       "@deepseek-ai/dsh-host-webserver": ["./packages/host/webserver/src"],
       "@deepseek-ai/dsh-client-ui-slots": ["./packages/client/ui-slots/src"],
+      "@deepseek-ai/dsh-client-ui-attachment": ["./packages/client/ui-attachment/src"],
       "@deepseek-ai/dsh-client-ui-primitives": ["./packages/client/ui-primitives/src"],
       "@deepseek-ai/dsh-client-schema-form": ["./packages/client/schema-form/src"],
       "@deepseek-ai/dsh-client-schema-form/invariant": ["./packages/client/schema-form/src/invariant.ts"],