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.module.css

@@ -0,0 +1,58 @@
+/* Transient top-center banner (DeepSeek Chat toast look): contrast fill,
+   inverted label, slide-in, then hold-and-fade. The fade delay/duration MUST
+   agree with HOLD_MS/FADE_MS in Toast.tsx: the component unmounts at their
+   sum, so a mismatched sheet either cuts the fade or leaves an invisible
+   banner blocking nothing. */
+
+.toast {
+  position: fixed;
+  top: 80px;
+  left: 50%;
+  /* Above the 1000 the image lightbox backdrop uses: a failure reported while
+     a preview is open must stay readable. */
+  z-index: 1100;
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  max-width: min(560px, calc(100vw - 48px));
+  padding: 12px 16px;
+  border-radius: 14px;
+  background: var(--dsw-alias-button-contrast-fill);
+  color: var(--dsw-alias-label-primary-inverted);
+  font-size: 14px;
+  line-height: 22px;
+  box-shadow: var(--dsw-shadow-lv3);
+  transform: translateX(-50%);
+  animation:
+    dsh-toast-in 160ms ease-out,
+    dsh-toast-fade 1000ms ease 3000ms forwards;
+}
+
+.icon {
+  display: grid;
+  place-items: center;
+  flex: none;
+  color: var(--dsw-alias-state-warn-label);
+}
+
+.text {
+  min-width: 0;
+}
+
+@keyframes dsh-toast-in {
+  from {
+    opacity: 0;
+    transform: translate(-50%, -6px);
+  }
+
+  to {
+    opacity: 1;
+    transform: translate(-50%, 0);
+  }
+}
+
+@keyframes dsh-toast-fade {
+  to {
+    opacity: 0;
+  }
+}