🎨 refactor(ui): redesign channel extra settings section in EditChannelModal

- 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

i18n/zh-cn.json

@@ -586,8 +586,7 @@
   "渠道额外设置": "渠道额外设置",
   "此项可选，用于配置渠道特定设置，为一个 JSON 字符串，例如：": "此项可选，用于配置渠道特定设置，为一个 JSON 字符串，例如：",
   "强制格式化": "强制格式化",
-  "强制格式化（只适用于OpenAI渠道类型）": "强制格式化（只适用于OpenAI渠道类型）",
-  "强制将响应格式化为 OpenAI 标准格式": "强制将响应格式化为 OpenAI 标准格式",
+  "强制将响应格式化为 OpenAI 标准格式（只适用于OpenAI渠道类型）": "强制将响应格式化为 OpenAI 标准格式（只适用于OpenAI渠道类型）",
   "思考内容转换": "思考内容转换",
   "将 reasoning_content 转换为 <think> 标签拼接到内容中": "将 reasoning_content 转换为 <think> 标签拼接到内容中",
   "透传请求体": "透传请求体",