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/app_es.arb

@@ -535,6 +535,22 @@
       }
     }
   },
+  "vacationImpactPausedLabel": "Pausa: {times}",
+  "@vacationImpactPausedLabel": {
+    "placeholders": {
+      "times": {}
+    }
+  },
+  "vacationImpactContinuesLabel": "Sigue sonando: {times}",
+  "@vacationImpactContinuesLabel": {
+    "placeholders": {
+      "times": {}
+    }
+  },
+  "vacationUpcomingSectionTitle": "PROGRAMADOS",
+  "vacationPastSectionTitle": "Rangos pasados",
+  "addVacationRangeCta": "Añadir rango",
+  "vacationNoActiveRangeHint": "No hay un rango de vacaciones activo ahora mismo.",
   "vacationsDefaultName": "Vacaciones",
   "newVacationRangeTitle": "Nuevo rango de vacaciones",
   "startField": "Inicio",