feat(sidebar): scan a single folder from the folder context menu (#1108)

The Refresh menu can scope a scan to one model root, but the folder a user is
looking at lives in the sidebar's unified tree, which merges every root into one
relative-path namespace. "Scan this folder" therefore addresses the folder, not a
root: the backend walks that relative path under every root that holds it, which
is also what makes the action safe while another drive is switched off.

Backend:
* GET /scan accepts `folder=<rel>` (alone or with `roots=`), rejected together
  with full_rebuild=true like the roots parameter. Validation reuses
  normalize_relative_folder(), extracted to module scope from ModelMoveService so
  the folder operations and the scan endpoint reject the same input (absolute
  paths, drive letters, `..` climbing) instead of each carrying its own copy.
* The reconcile summary carries `scope_label` (the folder) for a folder scope, so
  the result toast names the folder the user clicked instead of the roots it
  happens to live under; the completed WS payload carries it too.
* `folder` is a scope prefix exactly like a root: only that subtree is re-read or
  pruned, and an unreachable root keeps the entries that fall inside it.

Frontend:
* The sidebar folder context menu gains "Scan this folder" above "Check for
  updates in this folder" (they share the refresh divider); the entry is gated by
  the same supportsFolderManagement flag as the other folder operations.
* SidebarManager.scanFolder() resolves the node through the existing
  _resolveFolderCandidates() before doing anything: a folder no root holds any
  more explains itself ("no longer exists on disk") instead of scanning nothing,
  and an unresolvable multi-root node is refused rather than guessed.
* PageControls.refreshModels() and BaseModelApi.refreshModels() forward the folder
  scope, and _showRefreshSummary() prefers scope_label over the walked roots.

Verified live on the three-root sandbox with one drive switched off:
GET /scan?folder=pack000 walks drive-G and drive-Y, reports scope_label=pack000,
keeps drive-Z's 6 entries under that folder (kept_unreachable=6) and leaves all
420 models cached. 3704 passed, 7 skipped; frontend 1495 passed (150 files); vue
widgets 96 passed. The 2 new sidebar keys are [TODO: Translate] placeholders
pending the feature owner's go-ahead.

py/services/model_file_service.py

@@ -16,6 +16,33 @@ from ..services.pending_delete_service import PENDING_DELETE_DIR_NAME
 logger = logging.getLogger(__name__)
 
 
+def normalize_relative_folder(folder: str) -> str:
+    """Normalize a library-relative folder path, raising ``ValueError``.
+
+    Absolute paths (POSIX, drive-letter or UNC) and paths that climb out of the
+    library root are refused: both mean the caller is confused about which space
+    it is working in, and guessing would be worse than an error. Shared by the
+    folder operations and the scoped scan endpoint so both reject the same input.
+    """
+    raw = str(folder or "").strip()
+    if not raw:
+        raise ValueError("Folder path is required")
+
+    normalized = raw.replace("\\", "/")
+    if normalized.startswith("/") or (len(normalized) > 1 and normalized[1] == ":"):
+        raise ValueError("Folder path must be relative to a library root")
+
+    normalized = os.path.normpath(normalized)
+    if (
+        normalized == ".."
+        or normalized.startswith("../")
+        or normalized.startswith(".." + os.sep)
+    ):
+        raise ValueError("Folder path must stay inside the library root")
+
+    return normalized
+
+
 def _normalize_match_path(path: Any) -> str:
     """Normalize a path for set membership tests.
 
@@ -616,29 +643,8 @@ class ModelMoveService:
 
     @staticmethod
     def _normalize_relative_folder(folder: str) -> str:
-        """Normalize a library-relative folder path, raising ``ValueError``.
-
-        Absolute paths (POSIX, drive-letter or UNC) and paths that climb out of
-        the library root are refused: both mean the caller is confused about
-        which space it is working in, and guessing would be worse than an error.
-        """
-        raw = str(folder or "").strip()
-        if not raw:
-            raise ValueError("Folder path is required")
-
-        normalized = raw.replace("\\", "/")
-        if normalized.startswith("/") or (len(normalized) > 1 and normalized[1] == ":"):
-            raise ValueError("Folder path must be relative to a library root")
-
-        normalized = os.path.normpath(normalized)
-        if (
-            normalized == ".."
-            or normalized.startswith("../")
-            or normalized.startswith(".." + os.sep)
-        ):
-            raise ValueError("Folder path must stay inside the library root")
-
-        return normalized
+        """Normalize a library-relative folder path, raising ``ValueError``."""
+        return normalize_relative_folder(folder)
 
     async def delete_folder(self, folder_path: str, dry_run: bool = False) -> Dict[str, Any]:
         """Delete a model-free directory inside the model library roots.