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: [

test/pantallas/pantalla_ajustes_test.dart

@@ -306,6 +306,32 @@ void main() {
       expect(find.text('RECORDINGS & MUSIC'), findsOneWidget);
       expect(find.text('APPLICATION'), findsOneWidget);
     });
+
+    testWidgets(
+      'Issue 3 (feedback-pruebas): the gap between stacked settings groups '
+      'is 16, matching t4:523/534/541 -- not 12',
+      (tester) async {
+        setLargeSurface(tester);
+        _suppressListTileInkAssertion();
+        final estado = await crearEstado();
+        addTearDown(estado.dispose);
+
+        await tester.pumpWidget(buildAjustes(estado));
+        await pumpStable(tester);
+
+        for (final key in [
+          'ajustes-group-gap-1',
+          'ajustes-group-gap-2',
+          'ajustes-group-gap-3',
+        ]) {
+          expect(
+            tester.getSize(find.byKey(ValueKey(key))).height,
+            16,
+            reason: 't4:523/534/541 all draw a 16px gap between stacked groups',
+          );
+        }
+      },
+    );
   });
 }