fix(paths): keep business paths forward-slashed on Windows

Four places produced the host's separator (or a mixed path) where the app's
business form is forward-slashed, so on Windows the value could not be compared
with the one the rest of the app stores:

* `ModelScanner._calculate_folder` matched `file_path.startswith(root)` as raw
  strings, so every forward-slashed path resolved to the library root and a
  moved model was filed under the root instead of its folder. It now compares
  root identities and slices the relative part off.
* `normalize_relative_folder` ran `os.path.normpath`, and ntpath folds forward
  slashes back to backslashes: every folder parameter (scoped scan, create,
  rename, delete) came back host-spelled and mismatched the stored folders.
* `_format_model_name_for_comfyui` returned `os.path.relpath` unchanged, so the
  checkpoint/unet nodes and the metadata node emitted `unet\base.safetensors`
  on Windows -- a name no ComfyUI widget list matches.
* `resolve_centralized_dir_for_dir` passed a component that spans two
  directories ("<library>/<component>") whole to `os.path.join`, leaving a mixed
  path ("...\comfyui/loras-1a2b3c4d\sub") that no native path equals.

Also hands the rename re-key callback business paths, like every other absolute
path the move service returns.

py/utils/sidecar_paths.py

@@ -480,6 +480,19 @@ def _legacy_component(root_path: str) -> str:
     return f"{sanitize_path_component(os.path.basename(normalized))}-{digest}"
 
 
+def _join_component(root: str, component: str) -> str:
+    """Join a mirror component name onto *root* as a native path.
+
+    A component adopted from the pre-identity-map layout is reported as
+    ``<library>/<component>`` -- one *name* spanning two directories. Handing it
+    whole to ``os.path.join`` left a mixed path on Windows
+    (``...\\comfyui/loras-1a2b3c4d\\sub``), which no natively built path
+    compares equal to, so it has to be split first.
+    """
+
+    return os.path.join(root, *(part for part in component.split("/") if part))
+
+
 def _list_component_dirs(sidecar_root: str) -> Set[str]:
     """Return the mirror component directories present under ``sidecar_root``.
 
@@ -556,7 +569,7 @@ def _mirror_dir_overlap(sidecar_root: str, component: str, root: str) -> int:
 
     if not component:
         return 0
-    mirror_names = _subdir_names(os.path.join(sidecar_root, component))
+    mirror_names = _subdir_names(_join_component(sidecar_root, component))
     if not mirror_names:
         return 0
     root_names = _subdir_names(root)
@@ -958,7 +971,7 @@ def resolve_centralized_dir_for_dir(
     if not component:
         return None
 
-    parts = [root, component]
+    parts = [_join_component(root, component)]
     if rel_dir and rel_dir != os.curdir:
         parts.extend(
             sanitize_path_component(part)