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/modelos/alarma_musical.dart

@@ -277,6 +277,20 @@ class RangoVacaciones {
   }
 }
 
+/// Per-alarm vacation pause impact (design ADR-6, WU9). Produced by
+/// `EstadoAlarmas.impactoDeRango`, never persisted, never built from a
+/// second date-math implementation — see that method's own doc comment for
+/// the exact predicate it mirrors.
+class ImpactoVacaciones {
+  const ImpactoVacaciones({required this.pausadas, required this.noAfectadas});
+
+  /// `activa && !sonarEnVacaciones`.
+  final List<AlarmaMusical> pausadas;
+
+  /// `activa && sonarEnVacaciones`.
+  final List<AlarmaMusical> noAfectadas;
+}
+
 class ExcepcionAlarma {
   const ExcepcionAlarma({
     required this.alarmaId,