Refactor model configuration structure and update README

- Changed the configuration format to use a single 'models' list with entries containing 'url' and 'type'.
- Updated validation logic to ensure 'models' entries are correctly structured.
- Modified download logic to check for existing directories before downloading.
- Revised README to reflect new configuration format and usage instructions.

Signed-off-by: Daniel Henry <iamdanhenry@gmail.com>

model_downloader/download.py

@@ -129,7 +129,9 @@ def download_one(
         print(f"  Would download to: {dest_dir / filename_guess}")
         return True
 
-    dest_dir.mkdir(parents=True, exist_ok=True)
+    if not dest_dir.is_dir():
+        print(f"  Skip: Directory does not exist: {dest_dir}. Create models/<type> under your ComfyUI base.")
+        return False
 
     # Single GET with stream (Civitai redirects to S3 and often doesn't support HEAD)
     try: