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.client.json

@@ -46,6 +46,7 @@
     { "path": "./packages/compact/compact" },
     { "path": "./packages/client/ui-slots" },
     { "path": "./packages/client/schema-form" },
+    { "path": "./packages/client/ui-attachment" },
     { "path": "./packages/client/ui-primitives" },
     { "path": "./packages/client/web-react" },
     { "path": "./packages/client/modules" },