feat: add Pollinations BYOP device login

[voodoohop]

• Adds beecoder login using Pollinations device authorization with optional public client attribution.
• Handles pending, slow-down, denial, and expiry responses and stores the delegated key with 0600 permissions.
• Migrates chat proxying to gen.pollinations.ai/v1/chat/completions with Bearer authentication.
• Prevents the server from starting without a configured login and updates the README flow.

Checks:

• Mocked device authorization and error flows
• TypeScript syntax checks
• esbuild bundle validation
• CLI help validation
• git diff --check