Export current server list as a JSON download #1346

Background

#1343 wires the server list to ~/.mcp-inspector/mcp.json in the canonical { mcpServers: { ... } } format. Some users will want to download a snapshot to share, copy to another machine, or paste into another agent's config.

Sketch

• UI: an "Export" button on the Servers screen, next to Add (and the future Import — separate issue).
• Frontend only: GET /api/servers already returns the canonical MCPConfig JSON; the button triggers a Blob download named mcp.json.
• No backend changes needed — purely a client-side <a download> action.

Symmetric with the future "Import from other agents" button; the two can ship together if convenient.