feat(copilot): add GitHub Enterprise Server support (#2175)

* feat(copilot): add GitHub Enterprise Server support

* fix(copilot): address GHES PR review findings (P1 + 2×P2)

- P1: Use composite account ID (domain:user_id) for GHES to prevent
  cross-instance ID collisions; github.com keeps plain numeric ID for
  backward compatibilit
- P2-a: Use get_api_endpoint() for model list URL with automatic
  fallback to static URL when dynamic endpoint resolution fails
- P2-b: Add normalize_github_domain() as backend SSOT for domain
  normalization (lowercase, strip protocol/path/query, reject userinfo)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

src-tauri/src/proxy/providers/codex_oauth_auth.rs

@@ -203,6 +203,7 @@ impl From<&CodexAccountData> for GitHubAccount {
                 .unwrap_or_else(|| format!("ChatGPT ({})", &data.account_id)),
             avatar_url: None,
             authenticated_at: data.authenticated_at,
+            github_domain: "github.com".to_string(),
         }
     }
 }