feat(server): add opt-in gzip and WebSocket compression config

[efiten]

Summary

• Adds a compression config block to config.json (both fields default to false)
• compression.gzip: true — wraps the HTTP handler with a gzip middleware; compresses API responses and static assets when the client sends Accept-Encoding: gzip. WebSocket upgrade requests are passed through unmodified
• compression.websocket: true — enables permessage-deflate on the gorilla WebSocket upgrader

Motivation

Operators running behind a reverse proxy that does not compress upstream responses (e.g. nginx proxying to the Go server over a separate network leg) get no compression today. This adds an opt-in way to compress at the source.

Usage

"compression": {
  "gzip": true,
  "websocket": true
}

Test plan

• TestCompressionConfigDefaults — nil config returns false for both helpers
• TestCompressionConfigExplicitFalse — explicit false returns false
• TestCompressionConfigEnabled — explicit true returns true
• TestGZipMiddlewareCompresses — response is valid gzip when Accept-Encoding: gzip
• TestGZipMiddlewareSkipsNoAcceptEncoding — plain response when no header
• TestGZipMiddlewareSkipsWebSocket — WebSocket upgrade not compressed
• Existing test suite passes unchanged (zero-arg NewHub() calls untouched)

🤖 Generated with Claude Code

[efiten]

Duplicate of #934 which includes the e2e fix for the detached-element race.