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.

static/css/components/menu.css

@@ -67,6 +67,21 @@
     text-align: center;
 }
 
+/* Muted counter shown next to a menu label (e.g. how many empty folders the
+   "Show empty folders" toggle would reveal) */
+.context-menu-count {
+    color: var(--text-muted);
+    font-size: 12px;
+}
+
+/* The count keeps the label/tally muted even while the row is hovered, since
+   the accent background would otherwise wash the muted colour out. */
+.context-menu-item:hover .context-menu-count,
+.context-menu-item:focus-visible .context-menu-count {
+    color: var(--lora-text);
+    opacity: 0.8;
+}
+
 /* Section Headers */
 .context-menu-section-header {
     padding: 6px 12px 2px;