Summary

Adds MCP UI support to the Archestra Chat UI, enabling rich content rendering for MCP tool responses.

Changes

New files:

• McpUiRenderer component (mcp-ui-renderer.tsx) — renders MCP content types: text, image, and resource (with sandboxed HTML iframe support)
• sandbox_proxy.html — secure iframe proxy for HTML resource rendering
• mcp-ui-renderer.test.tsx — 16 tests covering parsing, rendering, edge cases, and graceful degradation

Modified files:

• chat-mcp-client.ts — forwards MCP UI content arrays to the frontend
• ool.tsx — integrates McpUiRenderer into tool response display
• chat-messages.tsx — integrates McpUiRenderer into chat message rendering

Testing

• 16/16 tests passing (vitest)
• Biome lint: clean (0 issues)
• TypeScript: compiles without errors

Closes #1301

/claim #1301