feat(auth): add OAuth2 support for xAI with PKCE and token persistence

- Implemented xAI OAuth2 integration with PKCE (Proof Key for Code Exchange) support.
- Added logic for token exchange, refresh, and persistent storage in JSON format.
- Created `xai` package with helpers for OAuth discovery, API token handling, and URL building.
- Introduced `XAIExecutor` for integrating xAI credentials into runtime HTTP requests.
- Added unit tests to validate OAuth flow, token persistence, and endpoint validation.

internal/tui/oauth_tab.go

@@ -24,6 +24,7 @@ var oauthProviders = []oauthProvider{
 	{"Codex (OpenAI)", "codex-auth-url", "🟩"},
 	{"Antigravity", "antigravity-auth-url", "🟪"},
 	{"Kimi", "kimi-auth-url", "🟫"},
+	{"xAI", "xai-auth-url", "⬛"},
 }
 
 // oauthTabModel handles OAuth login flows.
@@ -280,6 +281,8 @@ func (m oauthTabModel) submitCallback(callbackURL string) tea.Cmd {
 					providerKey = "antigravity"
 				case "kimi-auth-url":
 					providerKey = "kimi"
+				case "xai-auth-url":
+					providerKey = "xai"
 				}
 				break
 			}