fix(piece) : persist slot quantity on blur and send prix as string

- Save composant piece slot quantity via PATCH on blur
- Pass slotId through hierarchy and selection entries
- Send prix as string (not number) to match backend expectation
- Show quantity in view mode when > 1
- Allow quantity edit for all pieces (not just root-level)

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

app/pages/component/[id]/edit.vue

@@ -179,6 +179,7 @@
                     placeholder="Qté"
                     class="input input-bordered input-xs w-16 ml-auto"
                     @input="entry._definition.quantity = Math.max(1, entry._definition.quantity || 1)"
+                    @blur="saveSlotQuantity(entry)"
                   />
                 </li>
               </ul>
@@ -319,6 +320,7 @@ const {
   removeDocument,
   handleFilesAdded,
   submitEdition,
+  saveSlotQuantity,
   resolvePieceLabel,
   resolveProductLabel,
   resolveSubcomponentLabel,