Per-message copy + whole-conversation markdown export

[vahid-ahmadi]

Summary

• Adds a small Copy button next to the cost line on completed assistant turns. Copies only the user-facing prose (post-last-tool, chart placeholders rewritten as `[Chart]`) — what the user reads, not the working section.
• Adds a Download .md button in the chat header next to "Report issue". Exports the whole conversation as Markdown including the working section (as a `
  Details

  ` collapsible) and tool calls (Python code + output as fenced blocks). File is named `policyengine-chat-[-].md`.
• All export helpers (`extractFinalProse`, `messageToMarkdown`, `conversationToMarkdown`, `stripChartPlaceholders`, `formatPythonOutput`, `slugify`) are pure functions colocated above the component for now — easy to lift into `utils/` later if other surfaces need them.

Closes #43.

Test plan

• `docker-compose up` → ask a question that triggers `run_python`. Confirm a "Copy" button appears under the assistant answer once it finishes streaming. Click it → "Copied" feedback shows for 2s; pasting elsewhere yields the rendered markdown source of the answer (no working/CoT, no tool code).
• Click "Download .md" in the chat header → a `policyengine-chat-YYYY-MM-DD[-slug].md` file downloads. Open it: contains every user/assistant turn, the working section is a collapsible `
  Details

  `, Python code is a fenced ```python``` block, and chart placeholders read `[Chart]`.
• During streaming, the Copy button is hidden (only shows when `isComplete`).
• Download button is disabled while streaming.
• Plan-mode replies (no tools) — Copy gives the clarifying questions text; Download produces a clean export with no working section.

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Project	Deployment	Actions	Updated (UTC)
policyengine-uk-chat	Ready	Preview, Comment	May 6, 2026 11:30am

[github-actions]

Beta preview is ready.

• Frontend: open preview
• Backend: open backend