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/tests/toast.spec.tsx

@@ -0,0 +1,40 @@
+// @vitest-environment jsdom
+
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { cleanup, render } from '@testing-library/react'
+import { Toast } from '../src/Toast.tsx'
+
+afterEach(cleanup)
+
+describe('Toast', () => {
+  it('announces its text and reports done after the hold-and-fade lifetime', () => {
+    vi.useFakeTimers()
+    try {
+      const onDone = vi.fn()
+      const view = render(<Toast text="最多添加 50 张图片" icon={<svg data-testid="icon" />} onDone={onDone} />)
+      const banner = view.getByRole('alert')
+      expect(banner.textContent).toContain('最多添加 50 张图片')
+      expect(view.getByTestId('icon')).toBeTruthy()
+      vi.advanceTimersByTime(3999)
+      expect(onDone).not.toHaveBeenCalled()
+      vi.advanceTimersByTime(1)
+      expect(onDone).toHaveBeenCalledTimes(1)
+    } finally {
+      vi.useRealTimers()
+    }
+  })
+
+  it('renders without an icon and cancels its timer on unmount', () => {
+    vi.useFakeTimers()
+    try {
+      const onDone = vi.fn()
+      const view = render(<Toast text="plain" onDone={onDone} />)
+      expect(view.getByRole('alert').querySelector('[aria-hidden]')).toBeNull()
+      view.unmount()
+      vi.advanceTimersByTime(10_000)
+      expect(onDone).not.toHaveBeenCalled()
+    } finally {
+      vi.useRealTimers()
+    }
+  })
+})