feat(providers): add xAI (Grok) OAuth login support

[fisherdaddy]

Adds browser PKCE login against auth.x.ai with device-code fallback when the loopback callback port is busy, wires the credential into the OpenClaw auth-profiles store, and registers xAI as a selectable provider with its Grok model catalog.

Summary

Related Issue

Fixes #(issue number)

Changes Made

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation update
• Performance improvement
• Other (please describe):

Testing

• Tested locally
• Added new tests
• Updated existing tests
• Manual testing performed

Screenshots (if applicable)

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Electron-Specific Changes

• Changes to main process (src/main/)
• Changes to preload script (src/main/preload.ts)
• Changes to IPC communication
• Changes to window management
• None

Additional Notes