fix(agenda): one slot generator for professional and patient

The two calendars each built their own list of hours. The patient's
enforced a 3-hour booking lead time; the professional's did not, so at
15:00 an 08:00-18:00 agenda reported "3 libres" (15, 16, 17) that no
patient could actually take.

Both now go through SlotGenerator. The professional still sees the
near-term hours (blocking the next hour is legitimate) but they are
labelled "Sin reserva" and excluded from the "libres" count, so the
number on his agenda means what the patient sees.

Also in this pass:

- endOf() clamps the slot end at 23:59; a 120-minute slot booked at
  23:00 was sending "25:00" to the backend.
- ScheduleEntity.copyWith can set an hour back to null, so returning a
  day to jornada continua no longer keeps the split hours around.
- Removed the Firebase-era schedule map ('habilitado', 'range1Hour1')
  together with the dead ProfessionalEntity.fromDocument that fed it.
- Settings loads guard on mounted and swallow failures instead of
  calling setState after dispose.
- "Pedir cita" says what is missing instead of doing nothing.

Tests: 9 passing, including the clamp and the empty/inverted ranges.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/utils/service_day_param.dart

@@ -0,0 +1,9 @@
+/// The `day` value the services API expects: `yyyy-MM-dd`.
+///
+/// `DateTime.toString()` produces "2026-08-25 00:00:00.000Z", which happens to
+/// slip past the backend validator but is not canonical ISO 8601 and differs
+/// from what the web sends for the same table.
+String serviceDayParam(DateTime d) =>
+    '${d.year.toString().padLeft(4, '0')}-'
+    '${d.month.toString().padLeft(2, '0')}-'
+    '${d.day.toString().padLeft(2, '0')}';