feat(vacaciones): add vacation range manager screen

Add the Vacaciones manager screen per design ADR-6: an active-range
hero (name, days-remaining countdown, determinate progress bar, and a
per-alarm pause-impact line), a "PROGRAMADOS" upcoming-ranges list, an
"Add range" CTA, and a "Rangos pasados" history section. This is the
real destination WU8's Alarmas-root summary row pushes to, replacing
WU8's own temporary placeholder (_PantallaVacacionesTemporal, now
deleted).

EstadoAlarmas gains 4 pure query methods (rangoVacacionesActivo,
vacacionesProximas, vacacionesPasadas, impactoDeRango) -- read-only
over _alarmas/_vacaciones, no writes, no rescheduling, no native
bridge calls. impactoDeRango mirrors ServicioProgramacionAlarmas's own
pause predicate exactly, so the screen never disagrees with the
scheduler about which alarms are paused. ImpactoVacaciones joins
RangoVacaciones in alarma_musical.dart.

The add-range form (_EditorVacacionesSheet, _PickerButton) moved
verbatim from pantalla_alarmas.dart to its one remaining consumer.
estado_alarmas.dart's scheduling/snooze paths and the ringing screen's
dismiss guard are untouched; both test files pass unmodified.

New ARB keys (en/es only; other 11 locales are WU18's job):
vacationImpact{Paused,Continues}Label, vacationUpcomingSectionTitle,
vacationPastSectionTitle, addVacationRangeCta,
vacationNoActiveRangeHint.

lib/estado/estado_alarmas.dart

@@ -354,6 +354,69 @@ class EstadoAlarmas extends ChangeNotifier {
     await guardarVacaciones(nuevos);
   }
 
+  // ── Vacation queries (design ADR-6, WU9) ──────────────────────────────
+  // Four PURE queries: none writes, none reschedules, none touches the
+  // native bridge. Read-only over `_alarmas`/`_vacaciones`. Every method
+  // takes `{DateTime? ahora}` (clock injection) defaulting to
+  // `DateTime.now()` so tests can pass a fixed instant.
+
+  /// Currently-active vacation range, if today falls within one.
+  /// Delegates to the existing `RangoVacaciones.contiene(fecha)` — which
+  /// already handles the `activo` flag and day granularity — rather than
+  /// reimplementing date math (a second implementation is a second set of
+  /// off-by-one bugs). Callers derive "days remaining" themselves from the
+  /// returned range's `finDia`, the same way WU8's summary row already
+  /// does.
+  RangoVacaciones? rangoVacacionesActivo({DateTime? ahora}) {
+    final fecha = ahora ?? DateTime.now();
+    for (final rango in _vacaciones) {
+      if (rango.contiene(fecha)) return rango;
+    }
+    return null;
+  }
+
+  /// Ranges that have not yet started (`inicio > hoy`), soonest-first.
+  List<RangoVacaciones> vacacionesProximas({DateTime? ahora}) {
+    final fecha = ahora ?? DateTime.now();
+    final hoy = DateTime(fecha.year, fecha.month, fecha.day);
+    return _vacaciones.where((rango) => rango.inicioDia.isAfter(hoy)).toList()
+      ..sort((a, b) => a.inicioDia.compareTo(b.inicioDia));
+  }
+
+  /// Ranges whose end date has already passed (`fin < hoy`), most-recently-
+  /// ended first.
+  List<RangoVacaciones> vacacionesPasadas({DateTime? ahora}) {
+    final fecha = ahora ?? DateTime.now();
+    final hoy = DateTime(fecha.year, fecha.month, fecha.day);
+    return _vacaciones.where((rango) => rango.finDia.isBefore(hoy)).toList()
+      ..sort((a, b) => b.finDia.compareTo(a.finDia));
+  }
+
+  /// Per-alarm pause impact for [rango]. Mirrors
+  /// `ServicioProgramacionAlarmas`'s own pause predicate EXACTLY
+  /// (`servicio_programacion_alarmas.dart`:
+  /// `!alarma.sonarEnVacaciones && estaEnVacaciones(candidato, vacaciones)`)
+  /// — if these two ever diverge, the Vacaciones screen lies about which
+  /// alarms are paused. [rango] is accepted for API symmetry with the
+  /// other 3 queries above; the predicate itself needs no dates because it
+  /// only makes sense to call this for a range that IS currently active —
+  /// any alarm actually paused by it already has `sonarEnVacaciones ==
+  /// false`, which is exactly what the scheduler itself would have used to
+  /// skip that alarm's candidate occurrence.
+  ImpactoVacaciones impactoDeRango(RangoVacaciones rango) {
+    final pausadas = <AlarmaMusical>[];
+    final noAfectadas = <AlarmaMusical>[];
+    for (final alarma in _alarmas) {
+      if (!alarma.activa) continue;
+      if (alarma.sonarEnVacaciones) {
+        noAfectadas.add(alarma);
+      } else {
+        pausadas.add(alarma);
+      }
+    }
+    return ImpactoVacaciones(pausadas: pausadas, noAfectadas: noAfectadas);
+  }
+
   ExcepcionAlarma? ultimaExcepcionPara(String alarmaId) {
     final candidatas =
         _excepciones.where((e) => e.alarmaId == alarmaId).toList()