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.

scripts/verify-package-readme-model-experience.ts

@@ -61,6 +61,7 @@ const SENTENCE_MODEL_EXPERIENCE: Readonly<Record<string, SentenceContract>> = {
   'packages/client/modules': { kind: 'none', reason: 'Browser-side module-loading kernel machinery; registers nothing model-facing.' },
   'packages/client/test-runtime': { kind: 'none', reason: 'Browser-side test infrastructure (jsdom bench); registers nothing model-facing.' },
   'packages/client/ui-slots': { kind: 'none', reason: 'Browser-side UI plugin layer; registers nothing model-facing.' },
+  'packages/client/ui-attachment': { kind: 'none', reason: 'Browser-side UI plugin layer; registers nothing model-facing.' },
   'packages/client/ui-primitives': { kind: 'none', reason: 'Browser-side UI plugin layer; registers nothing model-facing.' },
   'packages/client/web-react': { kind: 'none', reason: 'Browser-side UI plugin layer; registers nothing model-facing.' },
   'packages/client/schema-form': { kind: 'none', reason: 'Browser-side form-rendering library; registers nothing model-facing.' },