Improve User Sign-in Flow

**Is your feature request related to a problem? Please describe.**
Our enterprise/EMU user sign-in flow currently involves multiple email/password login attempts before falling back to SSO. This has caused users to abandon the plugin without trying it and/or message GitHub admins for assistance.

**Describe the solution you'd like**
Present the user with a 'sign-in' picker with options for GitHub credentials, SSO, etc. rather than the current try/fail workflow.

**Additional Context**
Extension version: Actions - 0.25.8

未指定檔案或測試。先找出 TypeScript VS Code 擴充功能中目前的電子郵件/密碼和 SSO 登入流程，接著追蹤驗證選項的呈現方式。完成的標準是，使用者可以選擇 GitHub 憑證、SSO 或其他受支援的選項，而不必反覆嘗試登入失敗。

由索引模型根據 Issue 內容生成。