fix(server): add Content-Type headers for proxied static assets

[monotykamary]

Summary

• Fixes MIME type errors when loading the web interface via opencode web
• Adds automatic Content-Type header detection based on file extension when proxying responses from app.opencode.ai

Problem

When running opencode web, the browser console shows:

Failed to load module script: Expected a JavaScript-or-Wasm module script but the server responded with a MIME type of "". Strict MIME type checking is enforced for module scripts per HTML spec.

This happens because Cloudflare doesn't return Content-Type headers for static assets served from app.opencode.ai.

Solution

The proxy handler now detects file extensions (.js, .css, .wasm, etc.) and adds the appropriate Content-Type header to the response before returning it to the browser.

[monotykamary]

even though i'm a TUI guy, we gotta save the web @adamdotdevin @rekram1-node