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.

frontend/wailsjs/go/api/Service.d.ts

@@ -65,6 +65,8 @@ export function PinSession(arg1:string,arg2:boolean):Promise<void>;
 
 export function ReconnectSession(arg1:string):Promise<void>;
 
+export function RefreshGitHubRepos(arg1:string):Promise<Array<models.GitHubRepo>>;
+
 export function RegisterSecretResponseListener():Promise<void>;
 
 export function RenameSession(arg1:string,arg2:string):Promise<void>;