configurable slot duration: UI picker + genRanges stepMinutes

- Fix TimeOfDayExtension.add() to handle minute overflow via total-minutes math
- genRanges() now takes stepMinutes param (default 30, clamp 5-480)
- Profesional model: slotDurationMinutes field (parsed from slot_duration_minutes)
- ProfessionalFormProvider: saves slot duration alongside schedules on Guardar
- schedule_view: _SlotDurationPicker chip selector (15/20/30/45/60/90/120 min)
- ProfessionalCalendarView + CalendarView: pass slotDurationMinutes to genRanges

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/providers/professional_form_provider.dart

@@ -32,6 +32,7 @@ class ProfessionalFormProvider with ChangeNotifier {
     List<String>? specializationsPictures,
     Schedules? schedules,
     PaymentMethodEntity? paymentMethods,
+    int? slotDurationMinutes,
   }) {
     profesional = Profesional(
       id: id ?? profesional!.id,
@@ -53,6 +54,7 @@ class ProfessionalFormProvider with ChangeNotifier {
       specializationsPictures: specializationsPictures ?? profesional!.specializationsPictures,
       schedules: schedules ?? profesional!.schedules,
       paymentMethods: paymentMethods ?? profesional!.paymentMethods,
+      slotDurationMinutes: slotDurationMinutes ?? profesional!.slotDurationMinutes,
     );
     notifyListeners();
   }
@@ -106,9 +108,14 @@ class ProfessionalFormProvider with ChangeNotifier {
 
   Future<bool> updateProfesionalProfileScheduleInfo(String userId) async {
     try {
-      await _api.patch('/professionals/me/schedules', {
-        'schedules': profesional!.schedules.toSchedulesArray(),
-      });
+      await Future.wait([
+        _api.patch('/professionals/me/schedules', {
+          'schedules': profesional!.schedules.toSchedulesArray(),
+        }),
+        _api.patch('/professionals/me', {
+          'slot_duration_minutes': profesional!.slotDurationMinutes,
+        }),
+      ]);
     } catch (e) {
       NotificationsService.showSnackbar('Error al guardar: $e');
       return false;