feat(#152/#155): cleanup est_multiple + dashboard sans préfixe Admin (squash develop).

Suppression complète grossesse multiple / est_multiple (BDD, API, front).
Rename option C : widgets partagés et panels staff sous widgets/dashboard/,
AdminManagementWidget seul restant dans widgets/admin/.

backend/prisma/schema.prisma

@@ -33,7 +33,6 @@ model Child {
   dateOfBirth   DateTime
   photoUrl      String?
   photoConsent  Boolean   @default(false)
-  isMultiple    Boolean   @default(false)
   isUnborn      Boolean   @default(false)
   parentId      String
   parent        Parent    @relation(fields: [parentId], references: [id])