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/MachineComponentsCard.vue

@@ -28,11 +28,13 @@
         <div v-for="component in components" :key="component.id">
           <ComponentHierarchy
             :components="[component]"
-            :is-edit-mode="false"
+            :is-edit-mode="isEditMode"
             :show-delete="isEditMode"
             :collapse-all="collapsed"
             :toggle-token="collapseToggleToken"
+            @update="$emit('update-component', $event)"
             @edit-piece="$emit('edit-piece', $event)"
+            @custom-field-update="$emit('custom-field-update', $event)"
             @delete="$emit('remove-component', component.linkId || component.id)"
             @fill-entity="(linkId, typeId) => $emit('fill-entity', linkId, typeId)"
           />