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/composables/useMachineHierarchy.ts

@@ -237,6 +237,7 @@ export const buildMachineHierarchyFromLinks = (
         constructeurs: resolved?.constructeurs || [],
         documents: [],
         quantity,
+        slotId: def.slotId || definition.slotId || null,
         typePieceId: resolved?.typePieceId || definition.typePieceId || def.typePieceId || null,
         typePiece: resolved?.typePiece || null,
         parentComponentLinkId: machineComponentLinkId,