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-attachment/src/MessageImage.module.css

@@ -0,0 +1,53 @@
+.gallery {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 8px;
+  width: min(240px, 100%);
+}
+
+.gallery[data-align='end'] {
+  justify-content: flex-end;
+  align-self: flex-end;
+}
+
+.gallery[data-align='start'] {
+  justify-content: flex-start;
+  align-self: flex-start;
+}
+
+.frame {
+  display: grid;
+  flex: 0 0 auto;
+  place-items: center;
+  min-width: 44px;
+  min-height: 44px;
+  padding: 0;
+  overflow: hidden;
+  border: 1px solid var(--dsw-alias-border-l2-darkmode-thin);
+  border-radius: 16px;
+  background: var(--dsw-alias-interactive-bg-hover);
+  cursor: zoom-in;
+}
+
+.frame img {
+  display: block;
+  width: 100%;
+  height: 100%;
+  object-fit: contain;
+}
+
+.loading,
+.error {
+  color: var(--dsw-alias-label-tertiary);
+  font-size: 12px;
+  line-height: 18px;
+}
+
+.error {
+  max-width: 240px;
+  padding: 10px 12px;
+  border: 1px solid var(--dsw-alias-border-l2-darkmode-thin);
+  border-radius: 10px;
+  background: var(--dsw-alias-interactive-bg-hover-danger);
+  cursor: pointer;
+}