feat: Add extra folder paths support for LoRA Manager

Introduce extra_folder_paths feature to allow users to add additional
model roots that are managed by LoRA Manager but not shared with ComfyUI.

Changes:
- Add extra_folder_paths support in SettingsManager (stored per library)
- Add extra path attributes in Config class (extra_loras_roots, etc.)
- Merge folder_paths with extra_folder_paths when applying library settings
- Update LoraScanner, CheckpointScanner, EmbeddingScanner to include
  extra paths in their model roots
- Add comprehensive tests for the new functionality

This enables users to manage models from additional directories without
modifying ComfyUI's model folder configuration.

py/services/checkpoint_scanner.py

@@ -51,5 +51,16 @@ class CheckpointScanner(ModelScanner):
         return entry
 
     def get_model_roots(self) -> List[str]:
-        """Get checkpoint root directories"""
-        return config.base_models_roots
+        """Get checkpoint root directories (including extra paths)"""
+        roots: List[str] = []
+        roots.extend(config.base_models_roots or [])
+        roots.extend(config.extra_checkpoints_roots or [])
+        roots.extend(config.extra_unet_roots or [])
+        # Remove duplicates while preserving order
+        seen: set = set()
+        unique_roots: List[str] = []
+        for root in roots:
+            if root not in seen:
+                seen.add(root)
+                unique_roots.append(root)
+        return unique_roots