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/pages/machine/[id].vue

@@ -113,7 +113,7 @@
         @toggle-collapse="d.toggleAllComponents"
         @update-component="d.updateComponent"
         @edit-piece="d.updatePieceFromComponent"
-        @custom-field-update="d.updatePieceCustomField"
+        @custom-field-update="d.handleCustomFieldUpdate"
         @add-component="openAddModal('component')"
         @remove-component="async (id) => { await d.removeComponentLink(id); refreshVersions() }"
         @fill-entity="(linkId, typeId) => handleFillEntity(linkId, 'component', typeId)"
@@ -128,7 +128,7 @@
         :collapse-toggle-token="d.pieceCollapseToggleToken.value"
         @update-piece="d.updatePieceInfo"
         @edit-piece="d.editPiece"
-        @custom-field-update="d.updatePieceCustomField"
+        @custom-field-update="d.handleCustomFieldUpdate"
         @add-piece="openAddModal('piece')"
         @remove-piece="async (id) => { await d.removePieceLink(id); refreshVersions() }"
         @fill-entity="(linkId, typeId) => handleFillEntity(linkId, 'piece', typeId)"