fix: translate __orphan__ sentinel to null in bulk move; ungrouped units now fully bulk-operable

src/routes/admin/+page.svelte

@@ -125,12 +125,14 @@
     if (bulkAction === 'move') {
       if (!bulkMoveTargetGroupId || checkedUnitIds.size === 0) return;
       formError = null;
+      // '__orphan__' is the sentinel value for "move to ungrouped" (group: null)
+      const targetGroup = bulkMoveTargetGroupId === '__orphan__' ? null : bulkMoveTargetGroupId;
       const results = await Promise.all(
         [...checkedUnitIds].map((unitId) =>
           fetch(`/admin/api/units/${unitId}`, {
             method: 'PUT',
             headers: { 'Content-Type': 'application/json' },
-            body: JSON.stringify({ group: bulkMoveTargetGroupId })
+            body: JSON.stringify({ group: targetGroup })
           })
         )
       );