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/README.md

@@ -2,12 +2,16 @@
 
 English | [中文](README.zh.md)
 
-Pure React atoms (zero cordis): StateDot, DisclosureRow, ic_ds_* icons, Button/Pill/Menu/Modal/Input, the OnboardingSurface first-run takeover (body-portaled mask + opaque stage that holds `#root` inert for exactly its own lifetime), the markdown family (MessageText/MarkdownText/JsonBlock), the read-only JsonTree inspector, the `useAnchoredMaxHeight` hook that clamps a bottom-anchored overlay to the viewport space above its anchor (re-measured on resize, scroll, and a caller-supplied dependency), TerminalBlock, DiffBlock, ReadBlock, SearchBlock, and WebBlock.
+Pure React atoms (zero cordis): StateDot, DisclosureRow, ic_ds_* icons, Button/Pill/Menu/Modal/Input, the Toast transient banner, the OnboardingSurface first-run takeover (body-portaled mask + opaque stage that holds `#root` inert for exactly its own lifetime), the markdown family (MessageText/MarkdownText/JsonBlock), the read-only JsonTree inspector, the `useAnchoredMaxHeight` hook that clamps a bottom-anchored overlay to the viewport space above its anchor (re-measured on resize, scroll, and a caller-supplied dependency), TerminalBlock, DiffBlock, ReadBlock, SearchBlock, and WebBlock.
 
 ## Hover cards
 
 `HoverCard` keeps its portaled preview reachable across the anchor gap with a pointer-leave grace. A consumer may also pass `copyText`: the card then exposes button semantics for pointer and keyboard activation, includes that value after the `copyLabel` prefix in its accessible name, writes the exact value through the package clipboard helper, and temporarily replaces its content with `copiedLabel` only after the host accepts the write. A non-collapsed text selection intersecting the card suppresses pointer-click activation, while success feedback retains the original card height and clears when the card closes or after one second. `copyLabel` and `copiedLabel` are label props because this zero-cordis atom cannot read the application locale; omitting `copyText` preserves the read/select-only card. Historical rationale: [the archived hover-card copy note](../../../.agents/notes/archived/feature/2026-07-31-hover-card-click-copy.md).
 
+## Toast
+
+`Toast` is the transient top-center banner: it slides in, holds at full opacity for three seconds, fades over one second, then calls `onDone` so the owner can unmount it. It renders `role="alert"` with an optional leading icon slot and takes its copy as a required prop (zero-cordis: the owner localizes). Re-showing the same message requires a remount — owners key the element by a per-show sequence so an identical repeated message restarts the hold-and-fade cycle instead of silently reusing the faded banner. It layers above the ui-attachment image lightbox so a failure reported during a preview stays readable.
+
 ## Markdown rendering
 
 `MarkdownText` renders GFM and `$…$`, `$$…$$`, `\(…\)`, and `\[…\]` TeX math from untrusted assistant output through React elements, with math typeset by KaTeX and trusted commands disabled; block-level same-line `$$…$$` is display math, including `\tag{}`. A narrow micromark extension lets asterisk strong emphasis ending in punctuation close before adjacent CJK text, where prose normally omits the whitespace CommonMark requires; single-asterisk emphasis, non-CJK adjacency, escapes, code, and math retain upstream parsing. It omits raw HTML, neutralizes relative and non-HTTP(S)/mailto links, opens HTTP(S) links with safe external-link attributes, and renders absolute HTTP(S) images without a referrer; relative paths, absolute local paths, `file:` URLs, and unsupported schemes retain their alt text. Inline code whose complete value is an absolute HTTP(S) URL keeps its code styling and gains the same safe external anchor; commands, partial URLs, other schemes, and fenced code remain inert. An optional `fileMentions` resolver lets the owning view link inline code that names a real file: the token keeps code styling and gains a button wired to the resolved opener, with the resolver's accessible label and full-path `title`. The renderer never guesses at what looks like a path — an unresolved token stays inert, mentions apply to settled renders only (the streaming cache must not bake in handlers that could go stale), and a token inside an anchor stays inert because a button cannot nest there. While a reply streams, `MarkdownText` parses incrementally: all but the trailing two blocks freeze as cached React elements and only the source tail behind them re-parses per chunk, so per-chunk work tracks the tail instead of the whole reply ([mechanism and DOM-parity contract](../../../.agents/notes/implemented/architecture/2026-08-06-web-markdown-incremental-ast-renderer.md)). `MessageText` remains the literal-text primitive for user-authored content. `extractMarkdownPlainText` removes Markdown presentation markup for compact labels while preserving raw HTML as literal text. Element spacing, responsive images, tables, links, and inline code use the same `--dsw-alias-markdown-*` / `--dsw-font-markdown-*` tokens as deepsuite `@deepseek/md`. Fenced blocks render through `CodeBlock` (language banner, copy control, shiki for the registered grammars).