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/controls/PageControls.js

@@ -562,7 +562,7 @@ export class PageControls {
      * Refresh models list
      * @param {boolean} fullRebuild - Whether to perform a full rebuild
      */
-    async refreshModels(fullRebuild = false, { roots = null } = {}) {
+    async refreshModels(fullRebuild = false, { roots = null, folder = null } = {}) {
         if (!this.api) {
             console.error('API methods not registered');
             return;
@@ -572,10 +572,14 @@ export class PageControls {
         // model counts definitely did.
         this._scanScopeDetails = null;
         const scopedRoots = Array.isArray(roots) ? roots.filter(Boolean) : [];
+        const scopedFolder = typeof folder === 'string' && folder ? folder : null;
 
         try {
-            if (scopedRoots.length) {
-                await this.api.refreshModels(fullRebuild, { roots: scopedRoots });
+            if (scopedRoots.length || scopedFolder) {
+                await this.api.refreshModels(fullRebuild, {
+                    ...(scopedRoots.length ? { roots: scopedRoots } : {}),
+                    ...(scopedFolder ? { folder: scopedFolder } : {}),
+                });
             } else {
                 await this.api.refreshModels(fullRebuild);
             }