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.

tests/frontend/api/baseModelApi.refresh.test.js

@@ -153,8 +153,8 @@ describe('BaseModelApiClient.refreshModels scan progress', () => {
     };
   }
 
-  async function startRefresh(client, fullRebuild = false) {
-    const promise = client.refreshModels(fullRebuild);
+  async function startRefresh(client, fullRebuild = false, options = {}) {
+    const promise = client.refreshModels(fullRebuild, options);
     await vi.waitFor(() => {
       expect(FakeWebSocket.instances.length).toBe(1);
     });
@@ -392,6 +392,75 @@ describe('BaseModelApiClient.refreshModels scan progress', () => {
     await promise;
   });
 
+  it('requests a scoped scan when roots are passed', async () => {
+    global.fetch = vi.fn().mockResolvedValue({
+      ok: true,
+      json: async () => ({ status: 'success' }),
+    });
+
+    const client = await createClient();
+    await client.refreshModels(false, { roots: ['/mnt/a/loras', '/mnt/b/loras'] });
+
+    const [url] = global.fetch.mock.calls[0];
+    expect(url.searchParams.getAll('roots')).toEqual(['/mnt/a/loras', '/mnt/b/loras']);
+    expect(url.searchParams.get('full_rebuild')).toBe('false');
+  });
+
+  it('reports the scoped scan summary and the entries kept unreachable', async () => {
+    const fetchControl = mockFetchPending();
+    const client = await createClient();
+    const { promise } = await startRefresh(client, false, { roots: ['/mnt/a/loras'] });
+
+    fetchControl.resolveOk({
+      status: 'success',
+      scanned_roots: ['a/loras'],
+      added: 2,
+      removed: 1,
+      kept_unreachable: 5,
+      unavailable_paths: [
+        { path: '/mnt/g/loras', reason: 'root_unreachable', kept: 5 },
+      ],
+      skipped_roots: [{ path: '/mnt/g/loras', label: 'g/loras' }],
+    });
+    await promise;
+
+    expect(showToastMock).toHaveBeenCalledWith(
+      'toast.api.refreshCompleteScoped',
+      { scope: 'a/loras', added: 2, removed: 1 },
+      'success'
+    );
+    expect(showToastMock).toHaveBeenCalledWith(
+      'toast.api.refreshKeptUnreachable',
+      { count: '5', paths: 'g/loras' },
+      'info'
+    );
+  });
+
+  it('keeps the generic completion toast for a full-library scan', async () => {
+    const fetchControl = mockFetchPending();
+    const client = await createClient();
+    const { promise } = await startRefresh(client);
+
+    fetchControl.resolveOk({
+      status: 'success',
+      scanned_roots: ['a/loras', 'b/loras'],
+      added: 0,
+      removed: 0,
+    });
+    await promise;
+
+    expect(showToastMock).toHaveBeenCalledWith(
+      'toast.api.refreshComplete',
+      { action: 'Refresh' },
+      'success'
+    );
+    expect(showToastMock).not.toHaveBeenCalledWith(
+      'toast.api.refreshCompleteScoped',
+      expect.anything(),
+      expect.anything()
+    );
+  });
+
   it('shows the cancelled toast when the server reports cancellation', async () => {
     const fetchControl = mockFetchPending();
     const client = await createClient();