feat(calendar): add block/unblock slots for professionals

- ServicesProvider: add blockSlot() calling POST /services/block and
  unblockSlot() calling PATCH /services/:id/status with cancelled
- ProfessionalCalendarView:
  - Fix _services always-null bug: getServicesForProfessional() returns
    void; now reads from sp.services after it resolves
  - Differentiate self_booked (blocked) vs regular occupied slots
  - Show amber 'Bloqueado' state with lock icon for self_booked services
  - Show lock button on available slots → confirm dialog → blockSlot()
  - Show unlock button on blocked slots → confirm dialog → unblockSlot()
  - Reload calendar after block/unblock so UI reflects new state
  - Header stats now show separate counts: ocupadas / bloqueadas / libres
  - _ActionButton widget for reusable tap-target icon buttons

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/providers/services_provider.dart

@@ -51,6 +51,18 @@ class ServicesProvider extends ChangeNotifier {
     notifyListeners();
   }
 
+  // Blocks a slot on the professional's calendar by creating a self_booked service.
+  Future<void> blockSlot(String day, TimeOfDay time) async {
+    final h = time.hour.toString().padLeft(2, '0');
+    final m = time.minute.toString().padLeft(2, '0');
+    await _api.post('/services/block', {'day': day, 'range1_hour1': '$h:$m'});
+  }
+
+  // Unblocks a previously blocked slot by cancelling the self_booked service.
+  Future<void> unblockSlot(String serviceId) async {
+    await _api.patch('/services/$serviceId/status', {'status': 'cancelled'});
+  }
+
   getServiceForUser(String serviceId) async {
     try {
       isLoading = true;