fix(preview): resolve CORS error when setting CivitAI remote media as preview

- Add new endpoint POST /api/lm/{prefix}/set-preview-from-url to handle
  remote image downloads server-side, avoiding CORS issues
- Use rewrite_preview_url() to download optimized smaller images (450px width)
- Use Downloader service for reliable downloads with retry logic and proxy support
- Update frontend to call new endpoint instead of fetching images in browser

fixes #837

static/js/api/apiConfig.js

@@ -86,6 +86,7 @@ export function getApiEndpoints(modelType) {
 
         // Preview management
         replacePreview: `/api/lm/${modelType}/replace-preview`,
+        setPreviewFromUrl: `/api/lm/${modelType}/set-preview-from-url`,
 
         // Query operations
         scan: `/api/lm/${modelType}/scan`,