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/l10n/gen/app_localizations_it.dart

@@ -1026,6 +1026,29 @@ class AppLocalizationsIt extends AppLocalizations {
     return 'Próximo rango en $days días';
   }
 
+  @override
+  String vacationImpactPausedLabel(Object times) {
+    return 'Pausa: $times';
+  }
+
+  @override
+  String vacationImpactContinuesLabel(Object times) {
+    return 'Sigue sonando: $times';
+  }
+
+  @override
+  String get vacationUpcomingSectionTitle => 'PROGRAMADOS';
+
+  @override
+  String get vacationPastSectionTitle => 'Rangos pasados';
+
+  @override
+  String get addVacationRangeCta => 'Añadir rango';
+
+  @override
+  String get vacationNoActiveRangeHint =>
+      'No hay un rango de vacaciones activo ahora mismo.';
+
   @override
   String get vacationsDefaultName => 'Vacanze';