fix(storage): return empty array instead of null for file list

Initialize files slice with make() to ensure JSON serialization
returns [] instead of null when no files exist. Prevents frontend
TypeError when calling .map() on null.

backend/storage/storage.go

@@ -52,7 +52,7 @@ func (s *Storage) List() ([]string, error) {
 		return nil, fmt.Errorf("failed to read data directory: %w", err)
 	}
 
-	var files []string
+	files := make([]string, 0)
 	for _, entry := range entries {
 		if !entry.IsDir() && strings.HasSuffix(entry.Name(), ".md") {
 			files = append(files, strings.TrimSuffix(entry.Name(), ".md"))