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.

static/js/components/SidebarManager.js

@@ -2588,7 +2588,7 @@ export class SidebarManager {
         // collapsed afterwards so such a page shows the update check alone
         // instead of dangling separators.
         const supportsFolderManagement = this._supportsFolderManagement();
-        for (const action of ['create-subfolder', 'rename-folder', 'delete-folder']) {
+        for (const action of ['scan-folder', 'create-subfolder', 'rename-folder', 'delete-folder']) {
             const item = menu.querySelector(`[data-action="${action}"]`);
             if (item) {
                 item.style.display = supportsFolderManagement ? '' : 'none';
@@ -2684,6 +2684,9 @@ export class SidebarManager {
             case 'delete-folder':
                 this.showDeleteFolderModal(path);
                 break;
+            case 'scan-folder':
+                await this.scanFolder(path);
+                break;
             case 'check-folder-updates':
                 try {
                     await performFolderUpdateCheck(path);
@@ -2696,6 +2699,31 @@ export class SidebarManager {
         }
     }
 
+    /**
+     * Scan the clicked folder (and its subtree) only.
+     *
+     * The sidebar tree is a merged relative-path namespace, so the scan is
+     * addressed by the relative folder and the backend walks that path under
+     * every root that holds it — the same resolution the folder operations use.
+     * Nothing outside the folder is re-read or removed, which is what makes this
+     * safe to use while another drive is switched off.
+     */
+    async scanFolder(relativePath) {
+        const resolution = await this._resolveFolderCandidatesSafe(relativePath);
+        if (!resolution || resolution.candidates.length === 0) {
+            showToast('sidebar.scanFolderResult.missing', {}, 'warning');
+            return;
+        }
+
+        const controls = this.pageControls || this.lastPageControls;
+        if (!controls || typeof controls.refreshModels !== 'function') {
+            console.error('[SidebarManager] No page controls registered for a folder scan');
+            return;
+        }
+
+        await controls.refreshModels(false, { folder: relativePath });
+    }
+
     handleBreadcrumbClick(event) {
         const breadcrumbItem = event.target.closest('.sidebar-breadcrumb-item');
         const dropdownItem = event.target.closest('.breadcrumb-dropdown-item');