Implement OAuth device flow client

[Hozyne-OpenBak]

Part of #2

Implement OAuth 2.0 device flow for user authentication.

Tasks:

• Device authorization endpoint
• User code generation
• Polling for access token
• Error handling (timeout, denied, etc.)

Providers:

• OpenAI
• GitHub (Copilot)

Reference: https://datatracker.ietf.org/doc/html/rfc8628