HTTP compression middleware only supports gzip/deflate, missing zstd

[BYK]

Describe the bug

The HTTP compression middleware in compression.ts only supports gzip and deflate encoding. Modern browsers (Chrome 123+, Firefox 126+, Safari 18+) send zstd in Accept-Encoding and zstd provides ~30-40% better compression than gzip at similar speeds.

Steps to reproduce

1. Make a request to the OpenCode server with Accept-Encoding: zstd, gzip
2. Observe the response uses gzip, not zstd

Expected behavior

The server should negotiate zstd as the preferred encoding when the client supports it, falling back to gzip/deflate.

System info

• OpenCode version: v1.15.3
• OS: Linux