feat: add OpenAI OAuth support and GPT-5.4 aliases

[ThomsenDrake]

Stack position

This is PR 5/5 in the upstream contribution stack.

Depends on:

• Stack PR #13

GitHub only allows stacked base branches inside the same repository, so this review slice targets the previous branch on the fork. After earlier slices land, this can be retargeted upstream.

Summary

• add OpenAI OAuth as an alternate credential source for OpenAI-backed flows
• add upstream-safe gpt-5.4 aliases and listing support
• keep the desktop and frontend config surfaces in sync for the new auth path

Explicitly excluded

• OpenAI base URL changes
• Anthropic base URL changes
• Foundry or private endpoint defaults
• Brave, Tavily, Voyage, or other provider expansion

Notes

• targeted cleanup verification already passed for the OAuth and model-listing path before this PR was opened
• I have not rerun the full PR-specific test suite as part of PR creation