phsl / new-api

- Extract channel extra settings into a dedicated Card component for better visual hierarchy
- Replace custom gray background container with consistent Form component styling
- Simplify layout structure by removing complex Row/Col grid layout in favor of native Form component layout
- Unify help text styling by using extraText prop consistently across all form fields
- Move "Settings Documentation" link to card header subtitle for better accessibility
- Improve visual consistency with other setting cards by using matching design patterns

The channel extra settings (force format, thinking content conversion, pass-through body, proxy address, and system prompt) now follow the same design language as other configuration sections, providing a more cohesive user experience.

Affected settings:
- Force Format (OpenAI channels only)
- Thinking Content Conversion
- Pass-through Body
- Proxy Address
- System Prompt