Address GHE onboarding issues (#317205)

* Address GHE onboarding issues

* More polish

* Enhance GitHub Enterprise sign-in flow with improved UI state management and input validation

* Use InputBox for GHE onboarding entry

* Implement GHE instance input parsing and validation with unit tests

* Update onboarding button behavior based on user sign-in status

Co-authored-by: Copilot <copilot@github.com>

* Wrap overflow text

* Address PR review feedback

- Fix GHE_DOMAIN_REGEX to allow numbers (consistent with GHE_FULL_URI_REGEX)
- Use inputBox.onDidChange instead of raw DOM 'input' listener
- Remove hardcoded RGBA fallbacks in CSS that break light themes
- Extract duplicate error notification to _notifyEnterpriseSignInError helper
- Clear enterpriseSignInWatch when cancelling via back button or Escape
- Remove redundant .info CSS rule (identical to base rule)
- Add missing test for single-word slug with numbers

* Revert CSS rgba fallbacks, clear enterpriseSignInWatch in _nextStep

- Restore rgba() fallbacks in GHE CSS to match the rest of the file
  (these CSS variables may be undefined in some themes)
- Clear enterpriseSignInWatch in _nextStep when leaving sign-in step
  to prevent StopWatch leak when user clicks GHE then continues
  without signing in

* Address some PR feedback and cleanup listeners

* middle aligned arrow

---------

Co-authored-by: Copilot <copilot@github.com>
Co-authored-by: Logan Ramos <lramos15@gmail.com>
Co-authored-by: Logan Ramos <loganramos@microsoft.com>
Co-authored-by: Elijah King <elijahwilliamking@icloud.com>