feat: add headless login flow

[adrianorocha-dev]

Summary

• Adds ytstudio login --headless for server environments where the CLI cannot open a browser locally.
• Reuses the existing Desktop OAuth client and scopes by printing the authorization URL, accepting the failed loopback redirect URL from another browser, validating OAuth state, and exchanging the authorization code server-side.
• Keeps the existing ytstudio login local-server flow unchanged and updates setup docs for the headless path.

Test plan

• uv run pytest
• uv run ruff check src tests
• uv run ruff format --check src tests
• Manual install from fork branch on a VPS and verify the headless prompt/token exchange path reaches Google OAuth
• Full live YouTube channel login verification with maintainer OAuth credentials