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/shared/utils/structureSelectionUtils.ts

@@ -4,6 +4,7 @@ export type SelectionEntry = {
   requirementLabel: string
   resolvedName: string
   quantity?: number
+  slotId?: string
   _definition?: Record<string, any>
 }
 
@@ -62,6 +63,7 @@ export function collectStructureSelections(
         requirementLabel: resolvers.resolvePieceLabel(definition),
         resolvedName: catalogPiece?.name || selectedId,
         quantity: typeof definition?.quantity === 'number' ? definition.quantity : undefined,
+        slotId: isNonEmptyString(entry?.slotId) ? entry.slotId : undefined,
         _definition: definition,
       })
     })