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.

tests/frontend/regression/sidebarFolderContextMenu.test.js

@@ -2,11 +2,12 @@ import { describe, it, expect } from 'vitest';
 import { readFileSync } from 'fs';
 import path from 'path';
 
-// Regression guard for the sidebar folder context-menu layout: the update check
-// sits on top, the folder operations form a single group, and the destructive
-// entry stays last behind its own divider. SidebarManager gates those groups
-// per page and collapses the dividers when a group is hidden, so a reorder here
-// also changes what the recipes page shows.
+// Regression guard for the sidebar folder context-menu layout: the two refresh
+// actions (scan this folder, check for updates) sit on top, the folder
+// operations form a single group, and the destructive entry stays last behind
+// its own divider. SidebarManager gates those groups per page and collapses the
+// dividers when a group is hidden, so a reorder here also changes what the
+// recipes page shows.
 describe('Sidebar folder context menu layout', () => {
   const repoRoot = path.resolve(__dirname, '../../..');
   const html = readFileSync(
@@ -24,8 +25,9 @@ describe('Sidebar folder context menu layout', () => {
     return /data-action="([^"]+)"/.exec(rest)?.[1] || null;
   });
 
-  it('keeps the update check first and the folder operations grouped', () => {
+  it('keeps the refresh actions first and the folder operations grouped', () => {
     expect(sequence).toEqual([
+      'scan-folder',
       'check-folder-updates',
       'separator',
       'create-subfolder',