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/ImageLightbox.module.css

@@ -0,0 +1,34 @@
+.backdrop {
+  position: fixed;
+  inset: 0;
+  z-index: 1000;
+  display: grid;
+  place-items: center;
+  padding: 40px;
+  background: color-mix(in srgb, var(--dsw-alias-label-primary) 74%, transparent);
+}
+
+.image {
+  max-width: min(100%, 1600px);
+  max-height: calc(100vh - 80px);
+  object-fit: contain;
+  border-radius: 12px;
+  background: var(--dsw-specific-input-major);
+  box-shadow: var(--dsw-shadow-lv3);
+}
+
+.close {
+  position: fixed;
+  top: 20px;
+  right: 20px;
+  display: grid;
+  place-items: center;
+  width: 36px;
+  height: 36px;
+  border: 1px solid var(--dsw-alias-border-l2-darkmode-thin);
+  border-radius: 999px;
+  background: var(--dsw-specific-input-major);
+  color: var(--dsw-alias-label-primary);
+  font-size: 24px;
+  cursor: pointer;
+}