fix(alarmas): tighten the gap between stacked alarm cards to 10px

Tier 4 visual fidelity, audit 7.5 (t4 line 334): the prototype stacks
alarm cards with a 10px gap; the build used 12. Each gap is now keyed
per alarm id so a guard test can pin the exact value between two known
cards.

lib/pantallas/pantalla_alarmas.dart

@@ -63,7 +63,14 @@ class PantallaAlarmas extends StatelessWidget {
                 else
                   for (final alarma in estado.alarmas) ...[
                     _TarjetaAlarma(alarma: alarma),
-                    const SizedBox(height: 12),
+                    // 7.5 (Tier 4 visual fidelity): the prototype's gap
+                    // between stacked alarm cards is 10px (t4 line 334),
+                    // not 12. Keyed per alarm id so the guard test can
+                    // target the exact gap between two known cards.
+                    SizedBox(
+                      height: 10,
+                      key: ValueKey('alarm-card-gap-${alarma.id}'),
+                    ),
                   ],
                 _PanelVacaciones(estado: estado),
                 const SizedBox(height: 12),