feat(sidebar): show empty folders by default (#999)

The sidebar used the models-only folder list while the download and move
destination pickers list every directory, so a model downloaded into an
empty category folder did not appear in the sidebar at all. Empty folders
are also deliberate organization on disk, and a file-manager-shaped tree
that hides them is surprising. Default the preference to on.

Because the preference no longer gates fetching, both folder lists are
always loaded: the full list is the tree's single source of truth and the
empty-folder count, the models-only list is what "empty" is measured
against. That makes the view-options toggle a pure re-render, and lets the
new count decorate the "..." menu so the preference's effect is visible
without scanning the tree:

- empty-folder count shown next to the menu label, cleared when unknown
- the toggle is hidden entirely when there are no empty folders
- list view filters empty entries itself (the tree gets that for free
  from the backend, the flat list is now always loaded in full)
- creating a folder still re-enables the preference, since the folder the
  user just asked for would otherwise be invisible

Dim styling is decided by _isRenderedEmptyFolder() at render time; the
models-only set keeps its ancestor-expanded semantics for the delete
guard.

templates/components/context_menu.html

@@ -243,6 +243,7 @@
     </div>
     <div class="context-menu-item" data-action="toggle-empty-folders">
         <i class="fas fa-folder-open"></i> <span>{{ t('sidebar.showEmptyFolders') }}</span>
+        <span id="sidebarEmptyFoldersCount" class="context-menu-count"></span>
         <i class="fas fa-check check-indicator" style="margin-left:auto;display:none"></i>
     </div>
 </div>