feat: integrate 302.AI as sponsor provider (#126)

- Activate 302.AI option in init Step 1/4 and menu API config
- Auto-fill baseUrl (https://api.302.ai/cc), user only inputs API Key
- Show referral link (https://share.302.ai/oUDqQ6) in CLI
- Add sponsor banner + product intro to README.md and README.zh-CN.md
- Add 302.AI banner images (zh/en) to assets/sponsors/