feat(scanner): scan a single root and keep unreachable entries (#1108)

Refreshing had no way to say "scan only this drive": a user with three external
drives had to spin all of them up for every refresh, and switching a drive off
made the next refresh treat its whole library as deleted (rows pruned from the
memory cache and the SQLite cache, preview_url stripped on the next scroll).

Backend (py/services/model_scanner.py, py/config.py):
* ReconcileScope(roots, folder) + _reconcile_cache(scope=...): files inside the
  scope reconcile normally, everything outside is neither re-read nor removed.
  The folder half is plumbing for the sidebar entry in the next change.
* Path-level pruning guard: cached entries under a path this walk could not
  read are kept and reported instead of removed. Sources: a configured root that
  is not reachable (drive switched off while LM runs), a directory os.walk
  failed to enter (permissions / I/O error / Windows junction to an offline
  drive), and a known first-level symlink whose target is gone
  (Config.iter_path_mappings()).
* The recorded folder list is unioned instead of replaced whenever the scan did
  not verify every root, so a scoped scan cannot empty the sidebar.
* _reconcile_cache returns a summary (added / removed / repaired /
  scanned_roots / skipped_roots / unavailable_paths / kept_unreachable),
  exposed as ModelScanner.last_reconcile_summary, returned by
  BaseModelService.scan_models() and broadcast in the completed WS payload.
* _root_display_labels(): set-aware labels ("G: loras", "usb/loras") grown
  leftwards with real parent segments until unique, shared by the walk-progress
  line and the roots API.
* GET /scan accepts repeated `roots` (400 for unknown roots, 400 combined with
  full_rebuild=true); GET /roots gains root_details (label / reachable / cached
  count) while `roots` stays a plain path list for existing callers.
* serve_preview: a 404 no longer clears the cached preview_url when the file's
  own directory is unreachable - browsing the grid with a drive off used to
  strip preview references from the persistent cache.

Frontend:
* Refresh ▾ gains a "Scan one folder" section listing the page's roots with
  their cached counts; offline roots stay clickable and explain themselves; rows
  are wired by delegation (new static/js/components/controls/ScanScopeMenu.js).
* A scoped scan reports "Scanned <root>: N new, M removed"; a scan that kept
  entries reports "<N> models kept: <paths> not reachable".
* registerAPI() now injects the two cross-page passthroughs (fetchModelRoots and
  an argument-forwarding refreshModels) so a page facade cannot drop them: the
  first version rendered an empty menu and would have run a full refresh.
* createToastElement whitelists toast types, so a wrong `type` argument degrades
  to the info style instead of rendering an unstyled box.

Verified in a sandbox instance with three roots: a scoped scan walks only the
requested root (progress roots=0/1, 240 files); a full refresh with one root
offline reports kept_unreachable=60 and leaves all 420 models cached; /roots
reports the offline root with its cached count. 3699 passed, 7 skipped;
frontend 1488 passed (148 files); vue widgets 96 passed.

py/services/base_model_service.py

@@ -27,6 +27,7 @@ from ..utils.civitai_utils import build_civitai_model_page_url
 logger = logging.getLogger(__name__)
 
 if TYPE_CHECKING:
+    from .model_scanner import ReconcileScope
     from .model_update_service import ModelUpdateService
 
 
@@ -936,12 +937,25 @@ class BaseModelService(ABC):
         return self.scanner.get_hash_by_path(file_path)
 
     async def scan_models(
-        self, force_refresh: bool = False, rebuild_cache: bool = False
-    ):
-        """Trigger model scanning"""
-        return await self.scanner.get_cached_data(
-            force_refresh=force_refresh, rebuild_cache=rebuild_cache
+        self,
+        force_refresh: bool = False,
+        rebuild_cache: bool = False,
+        scope: Optional["ReconcileScope"] = None,
+    ) -> Optional[Dict[str, Any]]:
+        """Trigger model scanning, optionally restricted to a scope.
+
+        Returns the reconcile summary (added / removed / repaired / skipped
+        roots / kept entries) for incremental scans, ``None`` for a full
+        rebuild.
+        """
+        await self.scanner.get_cached_data(
+            force_refresh=force_refresh, rebuild_cache=rebuild_cache, scope=scope
         )
+        return self.scanner.last_reconcile_summary
+
+    def describe_model_roots(self) -> List[Dict[str, Any]]:
+        """Describe configured roots (label / reachability / cached count)."""
+        return self.scanner.describe_model_roots()
 
     async def get_model_info_by_name(self, name: str):
         """Get model information by name"""