Add assistant response copy action with copy format setting

[leonardoxr]

Summary

• add a dedicated copy action for completed assistant responses in the chat timeline
• add a client-only assistant response copy format setting with markdown and plain-text modes
• keep code-block copy behavior unchanged
• add markdown-to-plain-text serialization and regression coverage

Validation

• bun fmt
• bun lint
• bun typecheck
• bun run test -- src/lib/assistantMessageCopy.test.ts src/components/chat/MessagesTimeline.test.tsx in apps/web
• bun run test:browser -- src/components/ChatView.browser.tsx -t "copies the raw assistant markdown by default|copies assistant responses as plain text when the setting is enabled|keeps markdown code-block copy scoped to the code block" in apps/web

Assets

Chat before

Chat after

Settings before

Settings after

Interaction details

Issue: #1455

Closes #1455

---

Note

Medium Risk
Adds new clipboard-copy behavior and markdown parsing dependencies, and updates timeline virtualization height estimation based on a new client setting; regressions could affect chat rendering/scrolling or copy output.

Overview
Adds a copy button for completed assistant messages in the chat timeline, gated to non-streaming responses with copyable content, and wires it into ChatView via a new client-only assistantResponseCopyFormat setting (markdown default or plain-text).

Introduces assistantMessageCopy to resolve copy text (raw markdown or normalized plain text) using unified/remark-parse/remark-gfm with an LRU cache, and updates MessageCopyButton to support lazy text and accessible labels.

Updates timeline height estimation/virtualization to account for the new assistant action row and to re-measure when the copy format setting changes, and adds unit + browser tests plus a settings UI control to configure/reset the format.

^{Written by Cursor Bugbot for commit 4803895. This will update automatically on new commits. Configure here.}

Note

Add assistant response copy button with configurable markdown or plain-text copy format

• Adds a 'Copy response' button on completed assistant messages in MessagesTimeline, visible on hover/focus, that writes to the clipboard.
• Introduces a new assistantResponseCopyFormat client setting ('markdown' or 'plain-text', defaulting to 'markdown') configurable from the chat settings page.
• Adds assistantMessageCopy.ts which converts markdown to plain text using unified + remark-parse with custom renderers for GFM elements; results are LRU-cached (500 entries, ~5MB cap).
• Updates estimateTimelineMessageHeight to use a taller base height (96px vs 78px) for completed assistant messages that have copy-worthy content, keeping virtualization accurate.
• Behavioral Change: assistant message rows are taller by default for completed messages once the copy control is shown.

^{Macroscope summarized 4803895.}

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