fix(ajustes): group eyebrow outside the card, per-row accent icons

Audit 10.2 (t4:511): GrupoAjustes's eyebrow now sits OUTSIDE the
PluriGlassSurface, at title-tier (20px) padding -- it used to share
the card's own 16px content padding.

Audit 10.5 (t4:514/526): FilaAjuste gains an optional iconColor.
Wired on the two rows the prototype actually accents: AUDIO's
Ecualizador (brand cyan) and STATIONS' Grupos de favoritos (warmCoral).

Item 10.10 (two extra AUDIO rows -- "Calidad de streaming" and
"Reproduccion sin interrupciones") is NOT implemented: neither concept
has any backing state or service anywhere in this app today. Building
them for real means inventing two new persisted settings, and a
genuine streaming-quality preference would plausibly need to touch
the protected servicio_audio.dart to actually affect playback --
out of scope for a visual-fidelity pass. Decorative rows that open
nothing would be dead UI, which is worse than leaving the gap
disclosed.

test/pantallas/ajustes/widgets/fila_ajuste_test.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:pluriwave/pantallas/ajustes/widgets/fila_ajuste.dart';
 import 'package:pluriwave/tema/pluriwave_theme.dart';
+import 'package:pluriwave/widgets/pluri_glass_surface.dart';
 
 /// S8 (Tier 1 visual fidelity): the prototype puts a trailing "current
 /// value" on nearly every settings row, 13px `rgba(242,247,250,.55)` (t4
@@ -122,4 +123,81 @@ void main() {
       expect(divider.indent, 47);
     },
   );
+
+  testWidgets('visual fidelity (audit 10.5): an explicit iconColor tints the '
+      'leading icon (t4 lines 514/516/522 -- equalizer/hd/folder accents)', (
+    tester,
+  ) async {
+    await tester.pumpWidget(
+      host(
+        FilaAjuste(
+          icon: Icons.equalizer_rounded,
+          titulo: 'Ecualizador base',
+          iconColor: const Color(0xFF21D4D9),
+          onTap: () {},
+        ),
+      ),
+    );
+
+    final icon = tester.widget<Icon>(find.byIcon(Icons.equalizer_rounded));
+    expect(icon.color, const Color(0xFF21D4D9));
+  });
+
+  testWidgets(
+    'visual fidelity (audit 10.5): omitting iconColor keeps the default '
+    '(unchanged pre-10.5 behaviour)',
+    (tester) async {
+      await tester.pumpWidget(
+        host(
+          FilaAjuste(
+            icon: Icons.language_rounded,
+            titulo: 'Idioma',
+            onTap: () {},
+          ),
+        ),
+      );
+
+      final icon = tester.widget<Icon>(find.byIcon(Icons.language_rounded));
+      expect(icon.color, isNull);
+    },
+  );
+
+  testWidgets(
+    'visual fidelity (audit 10.2): the group eyebrow sits OUTSIDE the '
+    'card, at title-tier (20px) padding -- not inside the '
+    'PluriGlassSurface (t4 line 511)',
+    (tester) async {
+      _suppressListTileInkAssertion();
+      await tester.pumpWidget(
+        host(
+          GrupoAjustes(
+            titulo: 'AUDIO',
+            filas: [
+              FilaAjuste(
+                icon: Icons.equalizer_rounded,
+                titulo: 'Uno',
+                onTap: () {},
+              ),
+            ],
+          ),
+        ),
+      );
+
+      expect(
+        find.ancestor(
+          of: find.text('AUDIO'),
+          matching: find.byType(PluriGlassSurface),
+        ),
+        findsNothing,
+        reason: 'the eyebrow must not be a descendant of the card',
+      );
+
+      final eyebrowLeft = tester.getTopLeft(find.text('AUDIO')).dx;
+      expect(
+        eyebrowLeft,
+        20,
+        reason: 'S5 title tier -- PluriLayout.titleHorizontal',
+      );
+    },
+  );
 }