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/Toast.tsx

@@ -0,0 +1,37 @@
+import { useEffect } from 'react'
+import type { ReactNode } from 'react'
+import css from './Toast.module.css'
+
+/** Full-opacity hold before the fade starts. Must agree with the stylesheet's
+ * toast-fade delay (Toast.module.css) or the banner unmounts mid-fade. */
+const HOLD_MS = 3000
+/** Fade duration. Must agree with the stylesheet's toast-fade duration. */
+const FADE_MS = 1000
+
+/**
+ * Transient top-center banner: slides in, holds at full opacity, fades out,
+ * then reports done so the owner can unmount it. Re-showing the same text
+ * restarts the cycle when the owner remounts the component (key it by a
+ * per-show sequence).
+ *
+ * @param props.text - resolved banner copy; the owner passes localized text.
+ * @param props.icon - optional leading glyph (e.g. a warning icon).
+ * @param props.onDone - called once the fade completes; unmount the toast here.
+ * @returns the floating banner.
+ */
+export function Toast({ text, icon, onDone }: {
+  text: string
+  icon?: ReactNode
+  onDone: () => void
+}) {
+  useEffect(() => {
+    const timer = setTimeout(onDone, HOLD_MS + FADE_MS)
+    return () => { clearTimeout(timer) }
+  }, [onDone])
+  return (
+    <div className={css.toast} role="alert">
+      {icon !== undefined && <span className={css.icon} aria-hidden>{icon}</span>}
+      <span className={css.text}>{text}</span>
+    </div>
+  )
+}