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/utils/time_of_day_extension.dart

@@ -2,7 +2,8 @@ import 'package:flutter/material.dart';
 
 extension TimeOfDayExtension on TimeOfDay {
   TimeOfDay add({int hour = 0, int minute = 0}) {
-    return replacing(hour: this.hour + hour, minute: this.minute + minute);
+    final total = this.hour * 60 + this.minute + hour * 60 + minute;
+    return TimeOfDay(hour: (total ~/ 60) % 24, minute: total % 60);
   }
 
   int compareTo(TimeOfDay other) {