fix(ajustes): correct the gap between stacked settings groups

Issue 3 (feedback-pruebas): the prototype (t4:523/534/541) draws a
16px gap between the AUDIO/STATIONS/RECORDINGS/APPLICATION cards, not
12 -- a plain unwired literal that happened to collide with the
sectionGap/panelGap tokens' own value without actually citing the
prototype.

lib/pantallas/pantalla_ajustes.dart

@@ -147,7 +147,9 @@ class _AjustesContent extends StatelessWidget {
             ),
           ],
         ),
-        const SizedBox(height: 12),
+        // Issue 3 (feedback-pruebas): t4:523/534/541 all draw a 16px gap
+        // between these opaque, stacked settings groups -- not 12.
+        const SizedBox(height: 16, key: ValueKey('ajustes-group-gap-1')),
         GrupoAjustes(
           titulo: l10n.settingsGroupStationsTitle,
           filas: [
@@ -202,7 +204,7 @@ class _AjustesContent extends StatelessWidget {
             ),
           ],
         ),
-        const SizedBox(height: 12),
+        const SizedBox(height: 16, key: ValueKey('ajustes-group-gap-2')),
         GrupoAjustes(
           titulo: l10n.settingsGroupRecordingsTitle,
           filas: [
@@ -250,7 +252,7 @@ class _AjustesContent extends StatelessWidget {
             ),
           ],
         ),
-        const SizedBox(height: 12),
+        const SizedBox(height: 16, key: ValueKey('ajustes-group-gap-3')),
         GrupoAjustes(
           titulo: l10n.settingsGroupApplicationTitle,
           filas: [