feat(sort): enable versions_count sort in non-grouped mode

Sort by Most/Fewest versions first now works when Group by model is off.

- Backend: group items by modelId (respecting version_grouping setting),
  count versions per group, sort groups by count, expand groups with
  versions sorted by version id descending
- CSS: remove rule that hid the sort option in non-grouped mode
- Tests: add 3 tests covering desc, asc, and same_base variants

static/css/style.css

@@ -60,7 +60,4 @@
     margin-bottom: var(--space-2);
 }
 
-/* Hide versions_count sort option when group-by-model is off */
-body:not(.group-by-model) .sort-option-versions-count {
-    display: none;
-}
+/* ---------- reused from shared styles ---------- */