Feature request: host static React apps (SPA support)

[jiashuoz]

Users should be able to host a built React app (static HTML/JS/CSS) as an AgentDrive artifact.

Requirements:

• Serve a build output directory (index.html + assets) with correct MIME types and caching headers.
• SPA fallback: unknown paths under the app must serve index.html (client-side routing), not 404.
• Works with custom domains (see Feature request: custom domains (bring-your-own-domain) for hosted artifacts #3) so users can serve the app on their own domain.

Out of scope (for now): Next.js SSR or any server-side runtime - this is static hosting only.