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/model/definitionOverrides.ts

@@ -4,7 +4,7 @@ export interface DefinitionOverridePayload {
   name?: string
   reference?: string
   constructeurIds?: string[]
-  prix?: number
+  prix?: string
 }
 
 export const sanitizeDefinitionOverrides = (definition: any): DefinitionOverridePayload | null => {
@@ -41,7 +41,7 @@ export const sanitizeDefinitionOverrides = (definition: any): DefinitionOverride
   if (definition.prix !== undefined && definition.prix !== null && definition.prix !== '') {
     const parsed = Number(definition.prix)
     if (!Number.isNaN(parsed)) {
-      payload.prix = parsed
+      payload.prix = String(parsed)
     }
   }