fix(alarma-sonando): make the snooze block's vertical gaps uniform

Issue 3 (feedback-pruebas): t4:427 wraps the POSPONER eyebrow, the
snooze tiles and the Stop button in a single flex column with a
uniform gap:12 -- this screen carried a 10/14 pair instead, matching
neither the prototype nor each other.

The dismiss-guard test (protected, untouched) only asserts behaviour
via find.text/find.byType, so this pure value change is safe against
it -- re-verified empty diff after this commit.

lib/pantallas/pantalla_alarma_sonando.dart

@@ -339,14 +339,18 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
                       ],
                     ),
                   ),
-                  const SizedBox(height: 10),
+                  // Issue 3 (feedback-pruebas): t4:427 wraps POSPONER's
+                  // eyebrow, the snooze tiles and Stop in a `gap:12` flex
+                  // column -- the same 12 on both sides, not the 10/14 pair
+                  // this used to carry.
+                  const SizedBox(height: 12),
                   _FilaSnoozeFija(
                     alarma: alarma,
                     l10n: l10n,
                     tokens: tokens,
                     onPosponer: _posponer,
                   ),
-                  const SizedBox(height: 14),
+                  const SizedBox(height: 12),
                   SizedBox(
                     width: double.infinity,
                     child: FilledButton.icon(