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/models/profesional.dart

@@ -23,6 +23,7 @@ class Profesional {
   final List<String> specializationsPictures;
   final Schedules schedules;
   final PaymentMethodEntity paymentMethods;
+  final int slotDurationMinutes;
 
   const Profesional({
     required this.id,
@@ -44,6 +45,7 @@ class Profesional {
     required this.specializationsPictures,
     required this.schedules,
     required this.paymentMethods,
+    this.slotDurationMinutes = 30,
   });
 
   static Profesional empty() => Profesional(
@@ -148,6 +150,7 @@ class Profesional {
       specializationsPictures: specPics,
       schedules: parsedSchedules,
       paymentMethods: pm,
+      slotDurationMinutes: (doc['slot_duration_minutes'] as int?) ?? 30,
     );
   }
 
@@ -171,6 +174,7 @@ class Profesional {
     List<String>? specializationsPictures,
     Schedules? schedules,
     PaymentMethodEntity? paymentMethods,
+    int? slotDurationMinutes,
   }) {
     return Profesional(
       id: id ?? this.id,
@@ -194,6 +198,7 @@ class Profesional {
           specializationsPictures ?? this.specializationsPictures,
       schedules: schedules ?? this.schedules,
       paymentMethods: paymentMethods ?? this.paymentMethods,
+      slotDurationMinutes: slotDurationMinutes ?? this.slotDurationMinutes,
     );
   }