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/ui/views/calendar_view.dart

@@ -121,7 +121,8 @@ class _CalendarViewState extends State<CalendarView> {
                       children: [
                         ..._rangesItems(
                             _getScheduleFromNumDay(numDay, profesional),
-                            context),
+                            context,
+                            stepMinutes: profesional.slotDurationMinutes),
                         const SizedBox(height: 8),
                       ],
                     ),
@@ -157,7 +158,7 @@ class _CalendarViewState extends State<CalendarView> {
     }
   }
 
-  List<Widget> _rangesItems(ScheduleEntity? schedule, BuildContext context) {
+  List<Widget> _rangesItems(ScheduleEntity? schedule, BuildContext context, {int stepMinutes = 30}) {
     if (schedule == null) {
       return [
         const Padding(
@@ -187,6 +188,7 @@ class _CalendarViewState extends State<CalendarView> {
       List<TimeOfDay> ranges = TimeOfDayUtils.genRanges(
         schedule.range1Hour1!,
         schedule.range2Hour2!,
+        stepMinutes: stepMinutes,
       );
 
       return rangesItemList(ranges, _services, today, context);
@@ -203,10 +205,12 @@ class _CalendarViewState extends State<CalendarView> {
       List<TimeOfDay> ranges1 = TimeOfDayUtils.genRanges(
         schedule.range1Hour1!,
         schedule.range1Hour2!,
+        stepMinutes: stepMinutes,
       );
       List<TimeOfDay> ranges2 = TimeOfDayUtils.genRanges(
         schedule.range2Hour1!,
         schedule.range2Hour2!,
+        stepMinutes: stepMinutes,
       );
 
       return [