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-primitives/src/index.ts

@@ -23,6 +23,7 @@ export { FishLogo } from './FishLogo.tsx'
 export { BrandWordmark } from './BrandWordmark.tsx'
 export { Tooltip } from './Tooltip.tsx'
 export type { TooltipSide } from './Tooltip.tsx'
+export { Toast } from './Toast.tsx'
 export { writeClipboard } from './clipboard.ts'
 export { JsonTree } from './JsonTree.tsx'
 export type { JsonTreeProps, JsonTreeLabels } from './JsonTree.tsx'