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/components/controls/scanScopeMenu.test.js

@@ -0,0 +1,77 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+const { I18N_MODULE, MODULE } = vi.hoisted(() => ({
+  I18N_MODULE: new URL('../../../../static/js/utils/i18nHelpers.js', import.meta.url).pathname,
+  MODULE: new URL('../../../../static/js/components/controls/ScanScopeMenu.js', import.meta.url).pathname,
+}));
+
+vi.mock(I18N_MODULE, () => ({
+  translate: vi.fn((key, params, fallback) => {
+    if (!fallback) {
+      return key;
+    }
+    return Object.entries(params || {}).reduce(
+      (text, [name, value]) => text.replaceAll(`{${name}}`, value),
+      fallback
+    );
+  }),
+}));
+
+const { renderScanScopeMenu, resolveScanScopeTarget } = await import(MODULE);
+
+describe('ScanScopeMenu', () => {
+  let menu;
+
+  beforeEach(() => {
+    document.body.innerHTML = '<div id="refreshScopeMenu"></div>';
+    menu = document.getElementById('refreshScopeMenu');
+  });
+
+  it('renders one row per root with label, count and offline state', () => {
+    renderScanScopeMenu(menu, [
+      { path: '/mnt/a/loras', label: 'a/loras', reachable: true, models: 7 },
+      { path: '/mnt/b/loras', label: 'b/loras', reachable: false, models: 0 },
+    ]);
+
+    const rows = menu.querySelectorAll('.scan-root-item');
+    expect(rows.length).toBe(2);
+
+    expect(rows[0].dataset.action).toBe('scan-root');
+    expect(rows[0].dataset.root).toBe('/mnt/a/loras');
+    expect(rows[0].title).toBe('/mnt/a/loras');
+    expect(rows[0].querySelector('.scan-root-label').textContent).toBe('a/loras');
+    expect(rows[0].querySelector('.scan-root-count').textContent).toBe('7 models');
+    expect(rows[0].classList.contains('is-offline')).toBe(false);
+
+    expect(rows[1].classList.contains('is-offline')).toBe(true);
+    expect(rows[1].querySelector('.scan-root-offline').textContent).toBe('Offline');
+    // Offline rows stay clickable so the click can explain the state.
+    expect(rows[1].dataset.action).toBe('scan-root');
+  });
+
+  it('resolveScanScopeTarget reads path, label and offline state', () => {
+    renderScanScopeMenu(menu, [
+      { path: '/mnt/b/loras', label: 'b/loras', reachable: false, models: 0 },
+    ]);
+
+    expect(resolveScanScopeTarget(menu.querySelector('.scan-root-item'))).toEqual({
+      rootPath: '/mnt/b/loras',
+      label: 'b/loras',
+      offline: true,
+    });
+  });
+
+  it('skips malformed entries and replaces the previous list', () => {
+    renderScanScopeMenu(menu, [{ path: '/mnt/a' }, null, {}]);
+    expect(menu.querySelectorAll('.scan-root-item').length).toBe(1);
+
+    renderScanScopeMenu(menu, []);
+    expect(menu.querySelectorAll('.scan-root-item').length).toBe(0);
+  });
+
+  it('falls back to the path when a root has no label', () => {
+    renderScanScopeMenu(menu, [{ path: '/mnt/a/loras', reachable: true, models: 0 }]);
+
+    expect(menu.querySelector('.scan-root-label').textContent).toBe('/mnt/a/loras');
+  });
+});