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) {

lib/utils/time_of_day_utils.dart

@@ -2,13 +2,18 @@ import 'package:flutter/material.dart';
 import 'package:prosapp_web_app/utils/time_of_day_extension.dart';
 
 class TimeOfDayUtils {
-  static List<TimeOfDay> genRanges(TimeOfDay timeStart, TimeOfDay timeEnd) {
-    List<TimeOfDay> ranges = [];
-    TimeOfDay current = timeStart;
-    while (current.isBefore(timeEnd)) {
-      ranges.add(current);
-      // Sumar 2 horas al objeto DateTime
-      current = current.add(hour: 2);
+  static List<TimeOfDay> genRanges(
+    TimeOfDay timeStart,
+    TimeOfDay timeEnd, {
+    int stepMinutes = 30,
+  }) {
+    final List<TimeOfDay> ranges = [];
+    int curr = timeStart.hour * 60 + timeStart.minute;
+    final end = timeEnd.hour * 60 + timeEnd.minute;
+    final step = stepMinutes.clamp(5, 480);
+    while (curr < end) {
+      ranges.add(TimeOfDay(hour: curr ~/ 60, minute: curr % 60));
+      curr += step;
     }
     return ranges;
   }