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/src/entities/children.entity.ts

@@ -63,9 +63,6 @@ export class Children {
   @Column({ type: 'timestamptz', nullable: true, name: 'date_consentement_photo' })
   consent_photo_at?: Date;
 
-  @Column({ default: false, name: 'est_multiple', type: 'boolean' })
-  is_multiple: boolean;
-
   // Lien via table de jointure enfants_parents
   @OneToMany(() => ParentsChildren, pc => pc.child)
   parentLinks: ParentsChildren[];