WAN-96: clarify self-host websocket build docs

[haoxianhan]

Closes WAN-96

Summary

• Clarify that frontend API and WebSocket public env values are bundled at web build time for source-built images.
• Align self-host docs and web comments around default same-origin WebSocket derivation and reverse proxy upgrade handling.
• Add smoke guidance for localhost, LAN IP, single-domain reverse proxy, and separate-domain reverse proxy modes.

Validation

• bash -n scripts/selfhost-config.test.sh
• bash scripts/selfhost-config.test.sh
• docker compose config for prebuilt and source-build self-host paths
• pnpm --filter @multica/web test -- apps/web/config/runtime-urls.test.ts
• pnpm --filter @multica/web exec tsc --noEmit --pretty false
• pnpm --filter @multica/docs exec tsc --noEmit --pretty false
• git diff --check

Notes

• No production, cluster, secret, image tag, or release config changes.
• This PR publishes the already verified WAN-96 commit from the fork path after direct upstream push was denied.

[vercel]

@haoxianhan is attempting to deploy a commit to the IndexLabs Team on Vercel.

A member of the Team first needs to authorize it.