fix: match ItemModal stagesForType() to backend valid_stages_for() contract

stagesForType() returned OTHER_STAGES (including 'done') for the unset
type case, but backend stages.py::valid_stages_for(None) only allows
['new'] when type is unset. This let the UI offer 'Done' as a stage
option with no type selected, which the backend rejects with 422.

Also add a watch on type that resets stage to 'new' when the current
stage is no longer valid for the newly selected type, preventing a
stale invalid stage from being silently submitted.

frontend/src/components/ItemModal.vue

@@ -24,9 +24,17 @@ watch(() => props.item, (item) => {
 }, { immediate: true })
 
 function stagesForType() {
-  return type.value === 'donation' ? DONATION_STAGES : OTHER_STAGES
+  if (type.value === 'donation') return DONATION_STAGES
+  if (type.value === 'other') return OTHER_STAGES
+  return ['new']
 }
 
+watch(type, () => {
+  if (!stagesForType().includes(stage.value)) {
+    stage.value = 'new'
+  }
+})
+
 async function save() {
   const updated = await updateItem(props.item.id, {
     sender_id: senderId.value || null,