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/js/components/controls/PageControls.js

@@ -5,7 +5,9 @@ import { showToast, openCivitaiByMetadata, isTypingContext } from '../../utils/u
 import { eventManager } from '../../utils/EventManager.js';
 import { performModelUpdateCheck } from '../../utils/updateCheckHelpers.js';
 import { sidebarManager } from '../SidebarManager.js';
+import { getModelApiClient } from '../../api/modelApiFactory.js';
 import { initSortDropdown, applySortToSelect, randomizeSortValue } from './SortDropdown.js';
+import { renderScanScopeMenu, resolveScanScopeTarget } from './ScanScopeMenu.js';
 
 /**
  * PageControls class - Unified control management for model pages
@@ -82,7 +84,18 @@ export class PageControls {
      * @param {Object} api - API methods for the page
      */
     registerAPI(api) {
-        this.api = api;
+        this.api = {
+            ...api,
+            // These two are identical for every page and must stay in sync with
+            // the scan API: the refresh scope menu reads the root list, and a
+            // scoped refresh has to pass `roots` through. They are defined here
+            // rather than in each page facade so a page cannot silently drop
+            // them (which is exactly how the scope menu ended up empty).
+            fetchModelRoots: async () => getModelApiClient().fetchModelRoots(),
+            // Forward the exact arguments: an unscoped refresh must keep calling
+            // the client with a single argument.
+            refreshModels: async (...args) => getModelApiClient().refreshModels(...args),
+        };
         console.log(`API methods registered for ${this.pageType} page`);
         
         // Initialize sidebar manager after API is registered
@@ -233,7 +246,10 @@ export class PageControls {
                 });
                 
                 // Toggle current dropdown
-                dropdownGroup.classList.toggle('active');
+                const isOpen = dropdownGroup.classList.toggle('active');
+                if (isOpen) {
+                    this.loadScanScopeMenu();
+                }
             });
         });
         
@@ -248,6 +264,21 @@ export class PageControls {
             });
         }
 
+        // Per-root scan rows are rendered dynamically, so they are wired by
+        // delegation instead of a direct querySelector per item.
+        const scopeMenu = document.getElementById('refreshScopeMenu');
+        if (scopeMenu) {
+            scopeMenu.addEventListener('click', (e) => {
+                const item = e.target.closest('[data-action="scan-root"]');
+                if (!item) {
+                    return;
+                }
+                e.stopPropagation();
+                this.handleScanRootSelection(item);
+                document.querySelector('.dropdown-group.active')?.classList.remove('active');
+            });
+        }
+
         const checkUpdatesOption = document.getElementById('checkUpdatesMenuItem');
         if (checkUpdatesOption) {
             checkUpdatesOption.addEventListener('click', async (e) => {
@@ -266,6 +297,55 @@ export class PageControls {
         });
     }
 
+    /**
+     * Render the per-root scan entries of the refresh dropdown.
+     *
+     * Roots come from the backend (label, model count, reachability) so the menu
+     * always names a root exactly like the scan progress line does. The list is
+     * cached until the next scan finishes.
+     */
+    async loadScanScopeMenu() {
+        const menu = document.getElementById('refreshScopeMenu');
+        if (!menu || typeof this.api?.fetchModelRoots !== 'function') {
+            return;
+        }
+
+        if (this._scanScopeDetails) {
+            this.renderScanScopeMenu(this._scanScopeDetails);
+            return;
+        }
+        if (this._scanScopeLoading) {
+            return;
+        }
+
+        this._scanScopeLoading = true;
+        try {
+            const payload = await this.api.fetchModelRoots();
+            const details = Array.isArray(payload?.root_details) ? payload.root_details : [];
+            this._scanScopeDetails = details;
+            renderScanScopeMenu(menu, details);
+        } catch (error) {
+            console.error('Error loading model roots:', error);
+            menu.innerHTML = '';
+        } finally {
+            this._scanScopeLoading = false;
+        }
+    }
+
+    handleScanRootSelection(item) {
+        const { rootPath, label, offline } = resolveScanScopeTarget(item);
+        if (!rootPath) {
+            return;
+        }
+        if (offline) {
+            // showToast(key, params, type): the sentence must NOT be passed here,
+            // it used to land in the `type` slot and produced an unstyled toast.
+            showToast('toast.api.scanRootUnreachable', { scope: label }, 'info');
+            return;
+        }
+        this.refreshModels(false, { roots: [rootPath] });
+    }
+
     async handleCheckModelUpdates(menuItem) {
         if (this._updateCheckInProgress) {
             return;
@@ -482,14 +562,23 @@ export class PageControls {
      * Refresh models list
      * @param {boolean} fullRebuild - Whether to perform a full rebuild
      */
-    async refreshModels(fullRebuild = false) {
+    async refreshModels(fullRebuild = false, { roots = null } = {}) {
         if (!this.api) {
             console.error('API methods not registered');
             return;
         }
 
+        // Root labels / reachability may have changed with the scan, and the
+        // model counts definitely did.
+        this._scanScopeDetails = null;
+        const scopedRoots = Array.isArray(roots) ? roots.filter(Boolean) : [];
+
         try {
-            await this.api.refreshModels(fullRebuild);
+            if (scopedRoots.length) {
+                await this.api.refreshModels(fullRebuild, { roots: scopedRoots });
+            } else {
+                await this.api.refreshModels(fullRebuild);
+            }
             
             // Refresh sidebar after rebuild
             if (this.sidebarManager) {