feat(inscription AM #120): UI étape pro, photo unifiée, scripts et données test

- Panneau AM : validation NIR alignée API, date d’agrément requise côté app,
  capacité 1–10, n° agrément + date sur une ligne, ville/pays formatés au blur.
- Widget RegistrationPhotoSlot (cadre, croix) partagé avec les cartes enfant.
- AuthService : MIME PNG/JPEG pour la photo ; payload date_agrement.
- Scripts register-am-dubois / mansouri ; chemins tests/ressources/photos ;
  doc test-data + seed ; smoke curl inscription AM.

Made-with: Cursor

frontend/lib/screens/auth/am_register_step2_screen.dart

@@ -23,6 +23,7 @@ class AmRegisterStep2Screen extends StatelessWidget {
       birthCountry: registrationData.birthCountry,
       nir: registrationData.nir,
       agrementNumber: registrationData.agrementNumber,
+      agreementDate: registrationData.agreementDate,
       capacity: registrationData.capacity,
     );
 
@@ -43,6 +44,7 @@ class AmRegisterStep2Screen extends StatelessWidget {
           birthCountry: data.birthCountry,
           nir: data.nir,
           agrementNumber: data.agrementNumber,
+          agreementDate: data.agreementDate,
           capacity: data.capacity,
         );
         context.go('/am-register-step3');

frontend/lib/screens/auth/am_register_step4_screen.dart

@@ -204,6 +204,7 @@ class _AmRegisterStep4ScreenState extends State<AmRegisterStep4Screen> {
         birthCountry: data.birthCountry,
         nir: data.nir,
         agrementNumber: data.agrementNumber,
+        agreementDate: data.agreementDate,
         capacity: data.capacity,
         photoConsent: data.photoConsent,
       ),