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/components/sidebarManager.scanFolder.test.js

@@ -0,0 +1,142 @@
+import { describe, it, beforeEach, expect, vi } from 'vitest';
+
+const {
+  SIDEBAR_MANAGER_MODULE,
+  STORAGE_HELPERS_MODULE,
+  MODEL_API_FACTORY_MODULE,
+  I18N_MODULE,
+  BULK_MANAGER_MODULE,
+  UI_HELPERS_MODULE,
+  UPDATE_CHECK_MODULE,
+  STATE_MODULE,
+  MODAL_MANAGER_MODULE,
+} = vi.hoisted(() => ({
+  SIDEBAR_MANAGER_MODULE: new URL('../../../static/js/components/SidebarManager.js', import.meta.url).pathname,
+  STORAGE_HELPERS_MODULE: new URL('../../../static/js/utils/storageHelpers.js', import.meta.url).pathname,
+  MODEL_API_FACTORY_MODULE: new URL('../../../static/js/api/modelApiFactory.js', import.meta.url).pathname,
+  I18N_MODULE: new URL('../../../static/js/utils/i18nHelpers.js', import.meta.url).pathname,
+  BULK_MANAGER_MODULE: new URL('../../../static/js/managers/BulkManager.js', import.meta.url).pathname,
+  UI_HELPERS_MODULE: new URL('../../../static/js/utils/uiHelpers.js', import.meta.url).pathname,
+  UPDATE_CHECK_MODULE: new URL('../../../static/js/utils/updateCheckHelpers.js', import.meta.url).pathname,
+  STATE_MODULE: new URL('../../../static/js/state/index.js', import.meta.url).pathname,
+  MODAL_MANAGER_MODULE: new URL('../../../static/js/managers/ModalManager.js', import.meta.url).pathname,
+}));
+
+vi.mock(MODEL_API_FACTORY_MODULE, () => ({ getModelApiClient: vi.fn() }));
+vi.mock(I18N_MODULE, () => ({ translate: (key, _args, fallback) => fallback || key }));
+vi.mock(BULK_MANAGER_MODULE, () => ({ bulkManager: {} }));
+vi.mock(UI_HELPERS_MODULE, () => ({ showToast: vi.fn(), showActionToast: vi.fn() }));
+vi.mock(UPDATE_CHECK_MODULE, () => ({ performFolderUpdateCheck: vi.fn() }));
+vi.mock(MODAL_MANAGER_MODULE, () => ({
+  modalManager: { showModal: vi.fn(), closeModal: vi.fn() },
+}));
+
+const { SidebarManager } = await import(SIDEBAR_MANAGER_MODULE);
+const { showToast } = await import(UI_HELPERS_MODULE);
+
+function createApiClient(overrides = {}) {
+  return {
+    apiConfig: {
+      config: {
+        displayName: 'LoRA',
+        singularName: 'lora',
+        supportsMove: true,
+        supportsFolderManagement: true,
+      },
+    },
+    fetchModelRoots: vi.fn().mockResolvedValue({ roots: ['/models/loras', '/mnt/usb/loras'] }),
+    resolveFolder: vi.fn().mockResolvedValue({
+      success: true,
+      folder: 'pack',
+      candidates: [
+        { folder_path: '/models/loras/pack', root: '/models/loras', is_symlink: false },
+        { folder_path: '/mnt/usb/loras/pack', root: '/mnt/usb/loras', is_symlink: false },
+      ],
+    }),
+    ...overrides,
+  };
+}
+
+function createManager(apiClient) {
+  const manager = new SidebarManager();
+  manager.pageType = 'loras';
+  manager.apiClient = apiClient;
+  manager.renderFolderDisplay = vi.fn();
+  manager.renderEmptyState = vi.fn();
+  return manager;
+}
+
+describe('SidebarManager folder scan', () => {
+  beforeEach(() => {
+    localStorage.clear();
+    document.body.innerHTML = '';
+    showToast.mockClear();
+  });
+
+  it('scans the relative folder instead of guessing a root', async () => {
+    const apiClient = createApiClient();
+    const manager = createManager(apiClient);
+    const refreshModels = vi.fn().mockResolvedValue(undefined);
+    manager.pageControls = { refreshModels };
+
+    await manager.scanFolder('pack');
+
+    // The backend walks this relative path under every root that holds it, so
+    // the request carries no root at all.
+    expect(refreshModels).toHaveBeenCalledWith(false, { folder: 'pack' });
+    expect(showToast).not.toHaveBeenCalled();
+  });
+
+  it('falls back to the registered page controls after a re-init', async () => {
+    const apiClient = createApiClient();
+    const manager = createManager(apiClient);
+    manager.pageControls = null;
+    manager.lastPageControls = { refreshModels: vi.fn().mockResolvedValue(undefined) };
+
+    await manager.scanFolder('pack');
+
+    expect(manager.lastPageControls.refreshModels).toHaveBeenCalledWith(false, { folder: 'pack' });
+  });
+
+  it('explains a folder that no root holds any more', async () => {
+    const apiClient = createApiClient({
+      resolveFolder: vi.fn().mockResolvedValue({ success: true, folder: 'gone', candidates: [] }),
+    });
+    const manager = createManager(apiClient);
+    const refreshModels = vi.fn();
+    manager.pageControls = { refreshModels };
+
+    await manager.scanFolder('gone');
+
+    expect(refreshModels).not.toHaveBeenCalled();
+    expect(showToast).toHaveBeenCalledWith('sidebar.scanFolderResult.missing', {}, 'warning');
+  });
+
+  it('reports a failed resolution without scanning', async () => {
+    const apiClient = createApiClient({
+      resolveFolder: vi.fn().mockRejectedValue(new Error('offline')),
+      fetchModelRoots: vi.fn().mockResolvedValue({ roots: ['/models/loras', '/mnt/usb/loras'] }),
+    });
+    const manager = createManager(apiClient);
+    const refreshModels = vi.fn();
+    manager.pageControls = { refreshModels };
+
+    await manager.scanFolder('pack');
+
+    // Multi-root + no resolver: guessing a root is exactly what the folder
+    // operations refuse to do, and a scan must not guess either.
+    expect(refreshModels).not.toHaveBeenCalled();
+    expect(showToast).toHaveBeenCalledWith('sidebar.scanFolderResult.missing', {}, 'warning');
+  });
+
+  it('does nothing when no page controls are registered', async () => {
+    const apiClient = createApiClient();
+    const manager = createManager(apiClient);
+    manager.pageControls = null;
+    manager.lastPageControls = null;
+
+    await manager.scanFolder('pack');
+
+    expect(showToast).not.toHaveBeenCalled();
+  });
+});