fix(custom-fields) : context fields display, batch save, hierarchy propagation and UniqueEntity fix

- ComponentItem/PieceItem: DaisyUI divider, emit context-field-update for batch save
- CustomFieldDisplay: support editable/emit-blur/title/show-header props
- MachineComponentsCard/MachinePiecesCard: propagate custom-field-update events
- useMachineDetailCustomFields: pendingContextFieldUpdates + saveAllContextCustomFields
- useMachineDetailData: wire context field save into submitEdition
- useMachineDetailUpdates: only PATCH changed machine fields
- useMachineHierarchy: propagate contextCustomFields/Values from link to nodes
- componentStructure: include machineContextOnly in normalizeStructureForEditor
- Machine entity: convert empty reference to null, ignoreNull on UniqueEntity

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/app/components/machine/MachinePiecesCard.vue

@@ -34,6 +34,7 @@
             :toggle-token="collapseToggleToken"
             @update="$emit('update-piece', $event)"
             @edit="$emit('edit-piece', $event)"
+            @custom-field-update="$emit('custom-field-update', $event)"
             @delete="$emit('remove-piece', piece.linkId || piece.id)"
             @fill-entity="(linkId, typeId) => $emit('fill-entity', linkId, typeId)"
           />
@@ -66,6 +67,7 @@ defineEmits<{
   'toggle-collapse': []
   'update-piece': [piece: any]
   'edit-piece': [piece: any]
+  'custom-field-update': [fieldUpdate: any]
   'add-piece': []
   'remove-piece': [linkId: string]
   'fill-entity': [linkId: string, modelTypeId: string]