More specific error messages for GitHub token import network failures

What

Importing a GitHub token now gives more actionable error messages:

• Network errors are classified, and TLS-tunnel (ERR_PROXY_TUNNEL) failures now explain that the proxy refused the tunnel, via a new sslHint.
• Generic network errors during token import/web-setup now surface the sslHint and/or which proxy environment-variable source is in effect.
• Invalid-token (400) responses now parse the server's message and show it in the UI, appending guidance to add the repo scope.

Why

These changes turn generic, hard-to-diagnose network and token errors into specific messages that point at the actual cause, such as a proxy blocking the connection or a token missing a required scope.