feat(webui): redesign LLM settings section

- Rename "Experimental Setting" to "LLM Settings" with cleaner labels
- Replace jarring yellow warning banner with subtle themed notice
- Only show config fields when LLM is enabled (slide-fade transition)
- Add gpt-4o and gpt-4o-mini to model options
- Use scoped CSS instead of UnoCSS utility classes for the section
- Update i18n labels for both en and zh-CN

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

webui/types/config.ts

@@ -15,7 +15,7 @@ export type ProxyType = ['http', 'https', 'socks5'];
 /** 通知类型 */
 export type NotificationType = ['telegram', 'server-chan', 'bark', 'wecom'];
 /** OpenAI Model List */
-export type OpenAIModel = ['gpt-3.5-turbo'];
+export type OpenAIModel = ['gpt-4o', 'gpt-4o-mini', 'gpt-3.5-turbo'];
 /** OpenAI API Type */
 export type OpenAIType = ['openai', 'azure'];