feat(#156): création dossier AM staff — API + wizard dashboard.

Squash depuis develop : POST /assistantes-maternelles/dossier (actif +
mail MDP), AmDossierWizard create/review, validations inscription.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/lib/widgets/nir_text_field.dart

@@ -54,7 +54,7 @@ class NirTextField extends StatelessWidget {
       inputFontSize: inputFontSize,
       keyboardType: TextInputType.text,
       validator: validator ?? validateNir,
-      inputFormatters: [NirInputFormatter()],
+      inputFormatters: const [NirInputFormatter()],
       enabled: enabled,
       readOnly: readOnly,
       style: style,