Fix#1685: the Basic auth header for the OpenCode server was hardcoded
to use the username 'opencode', ignoring OPENCODE_SERVER_USERNAME. Users
who set a custom username got 401 errors because the server expected a
different credential.
Both call sites (web server auth-state-runtime.js and VS Code
extension opencode.ts) now read process.env.OPENCODE_SERVER_USERNAME
with a fallback to 'opencode' to preserve prior behavior.
Co-authored-by: Leonid Skorobogatyy <bash@opencode.itc.local>
