Add GitHub connection type — repo viewer with org grouping and auto-refresh

New protocol type 'github' for browsing repositories via the GitHub REST API.
Creates a non-terminal panel showing repos grouped by owner/org, sorted by
last commit date (newest first), with search filtering and auto-refresh.

Backend:
- ProtocolGitHub constant and GitHubConfig/GitHubRepo model structs
- New internal/githubclient package (REST API, pagination, token validation)
- Session manager connectGitHub() with periodic refresh goroutine
- API surface: RefreshGitHubRepos() for on-demand refresh
- Token stored via existing credential infrastructure (keychain/vault)

Frontend:
- GitHub tab in profile editor with API URL and token storage fields
- New github.js viewer component (grouped repos, badges, language dots)
- github.css with charcoal-themed repo cards and org headers
- Protocol colour tokens, tab badge, sidebar indicator for GitHub
- Event wiring for session:github:repos and session:github:error
- Cleanup on tab close

Supports GitHub Enterprise via configurable API base URL.

internal/api/api.go

@@ -618,6 +618,12 @@ func (s *Service) GetAppInfo() map[string]string {
 	}
 }
 
+// RefreshGitHubRepos triggers an on-demand refresh of GitHub repositories for
+// the given session. The updated list is also emitted as a Wails event.
+func (s *Service) RefreshGitHubRepos(sessionID string) ([]models.GitHubRepo, error) {
+	return s.sessions.FetchGitHubRepos(sessionID)
+}
+
 // TestConnection performs a basic TCP reachability check to the given host and port.
 // This verifies the network path without establishing a full protocol session.
 func (s *Service) TestConnection(host string, port int) (map[string]interface{}, error) {