Add a header button to the AI agent chat window that copies the active
conversation to the clipboard as Markdown, including the request
internals already persisted client-side — tool calls with their
input/output, per-message token usage, and finish/error info. No new
network call and no server/DB change: it serializes the already-loaded
persisted message rows.
- New util chat-markdown.ts (renamed from export-chat.ts): pure
buildChatMarkdown() serializer reusing the tool-parts helpers so tool
labels match the on-screen labels; fence() escapes embedded code
fences.
- ai-chat-window.tsx: Copy button (shown only for a saved chat with
loaded rows) using the project useClipboard hook; toggles a check
icon on success and shows the standard "Copied" notification. Drag is
unaffected (startDrag ignores button clicks).
- en-US: add "Copy chat" key, drop the obsolete "Export chat".
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