优化: 图片上传默认本地化,业务域名出代码

- 默认上传端点清空,未配置即本地base64内嵌,不再默认传业务文件服务
- 高级设置新增文件上传端点配置行,配置后启用上传(自定义服务协议不变)
- 同步清理注释内域名残留;小程序端同款硬编码已报备待后续处理

src/i18n/en/settings.ts

@@ -162,6 +162,11 @@ export default {
     descCustomPrompt: 'Appended to the AI system prompt to customize AI behavior',
     placeholderCustomPrompt: 'e.g.: You specialize in Rust development questions...',
 
+    // File upload endpoint
+    labelFileUploadBase: 'File upload endpoint',
+    descFileUploadBase: 'When set, chat images are uploaded to your own file service and turned into public URLs; leave empty to embed images locally (base64)',
+    placeholderFileUploadBase: 'e.g.: https://your-file-service.example.com',
+
     labelStreamingMd: 'Streaming Markdown rendering',
     descStreamingMd: 'Render code blocks/lists/headings during streaming (unclosed code blocks degrade to plain text to avoid flicker); when off, only plain text is shown during streaming and full formatting applies after generation completes',