Allow flat single path templates with no subfolder

Singles could not be saved as a flat file (e.g. "$artist - $title")
because the frontend blocked any template without a "/" and the
backend path builder treated an empty folder_path as falsy, falling
through to the hardcoded nested-folder structure.

Frontend: removed the must-include-slash validation for single
templates only (album templates still require it).
Backend: changed condition from `if folder_path and filename_base`
to `if filename_base` so an empty folder_path is handled correctly
as a flat drop into the transfer root.

web_server.py

@@ -17424,9 +17424,14 @@ def _build_final_path_for_track(context, spotify_artist, album_info, file_ext):
         }
 
         folder_path, filename_base = _get_file_path_from_template(template_context, 'single_path')
-        if folder_path and filename_base:
-            final_path = os.path.join(transfer_dir, folder_path, filename_base + file_ext)
-            os.makedirs(os.path.join(transfer_dir, folder_path), exist_ok=True)
+        if filename_base:
+            # folder_path may be '' for flat templates like "$artist - $title" (no subfolders)
+            if folder_path:
+                final_path = os.path.join(transfer_dir, folder_path, filename_base + file_ext)
+                os.makedirs(os.path.join(transfer_dir, folder_path), exist_ok=True)
+            else:
+                final_path = os.path.join(transfer_dir, filename_base + file_ext)
+                os.makedirs(transfer_dir, exist_ok=True)
             return final_path, True
         else:
             # Fallback

webui/static/script.js

@@ -5562,9 +5562,7 @@ function validateFileOrganizationTemplates() {
         if (singlePath.startsWith('/')) {
             errors.push('Single template cannot start with /');
         }
-        if (!singlePath.includes('/')) {
-            errors.push('Single template must include at least one folder (use / separator)');
-        }
+        // Note: single template is allowed to have no slash (flat file: "$artist - $title")
         if (singlePath.includes('//')) {
             errors.push('Single template cannot have consecutive slashes //');
         }