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/schedule_view.dart

@@ -11,6 +11,63 @@ import 'package:flutter/material.dart';
 import 'package:prosapp_web_app/ui/shared/widgets/schedule_day_tile.dart';
 import 'package:provider/provider.dart';
 
+// ── Slot duration options ──────────────────────────────────────────────────
+const _kSlotOptions = [
+  (15,  '15 min'),
+  (20,  '20 min'),
+  (30,  '30 min'),
+  (45,  '45 min'),
+  (60,  '1 hora'),
+  (90,  '1 h 30'),
+  (120, '2 horas'),
+];
+
+class _SlotDurationPicker extends StatelessWidget {
+  final int value;
+  final ValueChanged<int> onChanged;
+  const _SlotDurationPicker({required this.value, required this.onChanged});
+
+  @override
+  Widget build(BuildContext context) {
+    return Padding(
+      padding: const EdgeInsets.symmetric(vertical: 8),
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          const Text(
+            'Duración de cada cita',
+            style: TextStyle(fontSize: 14, fontWeight: FontWeight.w600),
+          ),
+          const SizedBox(height: 10),
+          Wrap(
+            spacing: 8,
+            runSpacing: 8,
+            children: _kSlotOptions.map((opt) {
+              final (mins, label) = opt;
+              final selected = value == mins;
+              return ChoiceChip(
+                label: Text(label),
+                selected: selected,
+                onSelected: (_) => onChanged(mins),
+                selectedColor: Colors.blue.shade400,
+                labelStyle: TextStyle(
+                  color: selected ? Colors.white : null,
+                  fontWeight: selected ? FontWeight.w700 : FontWeight.normal,
+                ),
+              );
+            }).toList(),
+          ),
+          const SizedBox(height: 4),
+          Text(
+            'Los usuarios verán bloques de $value min en el calendario.',
+            style: TextStyle(fontSize: 11, color: Colors.grey.shade600),
+          ),
+        ],
+      ),
+    );
+  }
+}
+
 class ScheduleView extends StatefulWidget {
   const ScheduleView({super.key});
 
@@ -540,9 +597,13 @@ class _ScheduleViewState extends State<ScheduleView> {
                         );
                       },
                     ),
-                    const Divider(
-                      height: 20,
+                    const Divider(height: 20),
+                    _SlotDurationPicker(
+                      value: profesional.slotDurationMinutes,
+                      onChanged: (v) => professionalFormProvider
+                          .copyProfesionalWith(slotDurationMinutes: v),
                     ),
+                    const Divider(height: 20),
                     Container(
                       margin: const EdgeInsets.only(top: 10),
                       child: ConstrainedBox(