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/routes/handlers/model_handlers.py

@@ -25,7 +25,10 @@ from ...services.connectivity_guard import (
     is_expected_offline_error,
 )
 from ...services.metadata_sync_service import MetadataSyncService
-from ...services.model_file_service import ModelMoveService
+from ...services.model_file_service import (
+    ModelMoveService,
+    normalize_relative_folder,
+)
 from ...services.model_scanner import ReconcileScope
 from ...services.preview_asset_service import PreviewAssetService
 from ...services.service_registry import ServiceRegistry
@@ -1143,7 +1146,8 @@ class ModelQueryHandler:
             requested_roots = [
                 value for value in request.query.getall("roots", []) if value
             ]
-            if requested_roots and full_rebuild:
+            raw_folder = (request.query.get("folder") or "").strip()
+            if (requested_roots or raw_folder) and full_rebuild:
                 return web.json_response(
                     {
                         "error": "Scoped scans are not supported with "
@@ -1152,7 +1156,13 @@ class ModelQueryHandler:
                     status=400,
                 )
 
-            scope = None
+            folder = None
+            if raw_folder:
+                try:
+                    folder = normalize_relative_folder(raw_folder)
+                except ValueError as exc:
+                    return web.json_response({"error": str(exc)}, status=400)
+
             if requested_roots:
                 configured = self._service.get_model_roots()
                 unknown = [root for root in requested_roots if root not in configured]
@@ -1160,7 +1170,15 @@ class ModelQueryHandler:
                     return web.json_response(
                         {"error": "Unknown model root(s)", "roots": unknown}, status=400
                     )
-                scope = ReconcileScope(roots=tuple(requested_roots))
+
+            # `folder` alone means "this relative folder in every root that has
+            # it" — the sidebar's unified tree carries no root identity.
+            scope = None
+            if requested_roots or folder:
+                scope = ReconcileScope(
+                    roots=tuple(requested_roots) if requested_roots else None,
+                    folder=folder,
+                )
 
             summary = await self._service.scan_models(
                 force_refresh=True, rebuild_cache=full_rebuild, scope=scope