fix(frontend): pre-select active unit when opening new equipment modal

When a unit is selected in the sidebar, the create-equipment modal now
pre-fills the unit dropdown with that unit. Previously it always reset
to empty, so newly created equipment got unit_id=null and was hidden by
the unit filter after save.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/js/equipment.js

@@ -94,6 +94,9 @@ export function closeEquipmentModal() {
 
 export function openCreateEquipmentModal() {
   resetEquipmentForm();
+  if (state.selectedUnitId && dom.equipmentUnitId) {
+    dom.equipmentUnitId.value = state.selectedUnitId;
+  }
   openEquipmentModal();
 }