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.

static/css/layout.css

@@ -700,3 +700,48 @@ body.sticky-controls .sticky-topbar {
         padding: 10px 0;
     }
 }
+
+/* Refresh dropdown: per-root scan scope (issue #1108). The list can hold one
+   entry per configured model root, so it scrolls instead of growing past the
+   viewport. Offline roots stay clickable (clicking explains the state) and are
+   only visually de-emphasised — unlike .dropdown-item.disabled, which swallows
+   the click via pointer-events: none. */
+.dropdown-separator {
+    height: 1px;
+    margin: 5px 0;
+    background-color: var(--border-color);
+}
+
+.dropdown-section-title {
+    padding: 4px 15px 2px;
+    font-size: 0.85em;
+    font-weight: 600;
+    color: var(--text-color);
+    opacity: 0.6;
+}
+
+.dropdown-scope-list {
+    max-height: 45vh;
+    overflow-y: auto;
+}
+
+.scan-root-item.is-offline {
+    cursor: default;
+    opacity: 0.6;
+}
+
+.scan-root-label {
+    margin-right: 12px;
+}
+
+.scan-root-count,
+.scan-root-offline {
+    float: right;
+    margin-left: 16px;
+    font-size: 0.9em;
+    opacity: 0.7;
+}
+
+.scan-root-item.is-offline .scan-root-offline {
+    opacity: 0.9;
+}