feat: add model_name and version_name placeholders to download paths, #552

Add support for {model_name} and {version_name} placeholders in download path templates. These new placeholders allow for more flexible and descriptive file organization by including the actual model name and version name in the download directory structure.

Changes include:
- Updated download_manager.py and utils.py to handle new placeholders
- Added placeholders to constants.js for UI reference
- Updated settings modal template to show available placeholders
- Added comprehensive tests to verify placeholder functionality

This enhancement provides users with more control over how downloaded models are organized on their file system.

static/js/utils/constants.js

@@ -117,7 +117,9 @@ export const DOWNLOAD_PATH_TEMPLATES = {
 export const PATH_TEMPLATE_PLACEHOLDERS = [
     '{base_model}',
     '{author}',
-    '{first_tag}'
+    '{first_tag}',
+    '{model_name}',
+    '{version_name}'
 ];
 
 // Default templates for each model type