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/.

frontend/lib/models/dossier_unifie.dart

@@ -185,7 +185,6 @@ class EnfantDossier {
   final String? dueDate;
   final String? photoUrl;
   final bool consentPhoto;
-  final bool estMultiple;
 
   EnfantDossier({
     required this.id,
@@ -197,7 +196,6 @@ class EnfantDossier {
     this.dueDate,
     this.photoUrl,
     this.consentPhoto = false,
-    this.estMultiple = false,
   });
 
   String get fullName => '${firstName ?? ''} ${lastName ?? ''}'.trim();
@@ -231,8 +229,6 @@ class EnfantDossier {
       photoUrl: resolvedPhoto,
       consentPhoto:
           json['consent_photo'] == true || json['consentPhoto'] == true,
-      estMultiple:
-          json['est_multiple'] == true || json['estMultiple'] == true,
     );
   }
 }

frontend/lib/models/enfant_admin_model.dart

@@ -12,7 +12,6 @@ class EnfantAdminModel {
   final String status;
   final String? photoUrl;
   final bool consentPhoto;
-  final bool isMultiple;
   final List<EnfantParentLink> parentLinks;
   /// Flag API #157 (sinon déduit de [parentLinks]).
   final bool? sansResponsable;
@@ -27,7 +26,6 @@ class EnfantAdminModel {
     required this.status,
     this.photoUrl,
     this.consentPhoto = false,
-    this.isMultiple = false,
     this.parentLinks = const [],
     this.sansResponsable,
   });
@@ -56,7 +54,6 @@ class EnfantAdminModel {
     String? status,
     String? photoUrl,
     bool? consentPhoto,
-    bool? isMultiple,
     List<EnfantParentLink>? parentLinks,
     bool? sansResponsable,
   }) {
@@ -70,7 +67,6 @@ class EnfantAdminModel {
       status: status ?? this.status,
       photoUrl: photoUrl ?? this.photoUrl,
       consentPhoto: consentPhoto ?? this.consentPhoto,
-      isMultiple: isMultiple ?? this.isMultiple,
       parentLinks: parentLinks ?? this.parentLinks,
       sansResponsable: sansResponsable ?? this.sansResponsable,
     );
@@ -111,8 +107,6 @@ class EnfantAdminModel {
       ),
       photoUrl: photoUrl,
       consentPhoto: consentPhoto,
-      isMultiple: _parseBool(json['is_multiple']) ||
-          _parseBool(json['est_multiple']),
       parentLinks: links,
       sansResponsable: sansResponsable,
     );
@@ -127,7 +121,6 @@ class EnfantAdminModel {
       if (birthDate != null && birthDate!.isNotEmpty) 'birth_date': birthDate,
       if (dueDate != null && dueDate!.isNotEmpty) 'due_date': dueDate,
       'consent_photo': consentPhoto,
-      'is_multiple': isMultiple,
     };
   }
 

frontend/lib/models/parent_user_registration_data.dart

@@ -30,7 +30,6 @@ class ChildData {
   String lastName;
   String dob; // Date de naissance ou prévisionnelle
   bool photoConsent;
-  bool multipleBirth;
   bool isUnbornChild;
   File? imageFile;
   CardColorVertical cardColor; // Nouveau champ pour la couleur de la carte
@@ -40,7 +39,6 @@ class ChildData {
     this.lastName = '',
     this.dob = '',
     this.photoConsent = false,
-    this.multipleBirth = false,
     this.isUnbornChild = false,
     this.imageFile,
     required this.cardColor, // Rendre requis dans le constructeur

frontend/lib/models/user_registration_data.dart

@@ -38,7 +38,6 @@ class ChildData {
   /// Valeurs API : `H`, `F`, `Autre` (GenreType backend). Vide tant que non choisi.
   String genre;
   bool photoConsent;
-  bool multipleBirth;
   bool isUnbornChild;
   File? imageFile;
   /// Octets de la photo (fiable à l’envoi API ; [imageFile] peut être absent sur le web).
@@ -55,7 +54,6 @@ class ChildData {
     this.dob = '',
     this.genre = '',
     this.photoConsent = false,
-    this.multipleBirth = false,
     this.isUnbornChild = false,
     this.imageFile,
     this.imageBytes,
@@ -70,7 +68,6 @@ class ChildData {
     String? dob,
     String? genre,
     bool? photoConsent,
-    bool? multipleBirth,
     bool? isUnbornChild,
     Object? imageFile = _unsetImage,
     Object? imageBytes = _unsetImageBytes,
@@ -84,7 +81,6 @@ class ChildData {
       dob: dob ?? this.dob,
       genre: genre ?? this.genre,
       photoConsent: photoConsent ?? this.photoConsent,
-      multipleBirth: multipleBirth ?? this.multipleBirth,
       isUnbornChild: isUnbornChild ?? this.isUnbornChild,
       imageFile: identical(imageFile, _unsetImage) ? this.imageFile : imageFile as File?,
       imageBytes: