Draft: Migrate OAuth flow to use device authorization grant method

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What does this MR do and why?

The change replaces the OAuth browser-based redirect flow with the device authorization grant flow, which provides a better auth experience. This removes the need to run a local HTTP server and simplifies token handling.

Key changes:

• Implement device authorization flow per GitLab OAuth2 spec
• Add polling mechanism for token retrieval
• Remove PKCE and redirect handling code

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