Add export chat history as JSON/Markdown

[cheapestinference]

Description

There is currently no way to export or download a chat session's history from the dashboard. Users who want to save conversations, share them, or analyze them offline have no built-in option to do so.

This issue is about adding an "Export" or "Download" button to the session/chat view that allows users to download the conversation in JSON format (raw structured data) and/or Markdown format (human-readable).

Where to look

• src/components/views/ChatView.tsx — The main chat view component. This is where the export button should be placed, likely in the header area or as a toolbar action. The chat messages are available as chatMessages (array of ChatMessage objects).
• src/components/views/SessionDetailView.tsx — The session detail wrapper that renders ChatView. An export button could alternatively be placed in this component's header/toolbar area.
• src/types/openclaw.ts — Defines the ChatMessage interface (line ~286) with fields like role, content, timestamp, model, runId, etc. This is the data structure you will be serializing.
• src/lib/utils.ts — Good place to add export helper functions (e.g., exportAsJSON(), exportAsMarkdown()).
• src/lib/browser-utils.ts — May already contain browser-related utilities; file download logic could go here.

Acceptance criteria

• A download/export button is visible in the session or chat view (icon button or dropdown with format options)
• Users can export the chat history as JSON (preserving all message metadata: role, content, timestamp, model) and as Markdown (formatted with ## User / ## Assistant headers and readable content)
• The exported file is named descriptively (e.g., chat-{agentName}-{sessionKey}-{date}.json) and triggers a browser file download