fix(chrome): drop the global AppBar, give each root its own title row

The prototype (t4) draws no global app bar anywhere: every root paints
a plain ~56px title row inside its own content instead (Alarmas line
325, Ajustes line 511, Explorar line 641). app.dart wrapped every tab
in PluriWaveScaffold(appBar: AppBar(title: Text(appTitle), ...)),
adding 56dp of chrome and a "PluriWave" title the prototype never
shows.

Add PluriRootHeader, a shared 56px title-row widget reused by all 5
roots. Extract app.dart's old _mostrarTimerDialog (only reachable from
the removed AppBar action) into a free function,
showPluriSleepTimerSheet, so every root's header can open the same
sheet directly and the sleep-timer feature stays reachable from every
tab with no behaviour change.

S1, Tier 1 visual-fidelity pass (audit id 2521).

lib/pantallas/pantalla_favoritos.dart

@@ -10,6 +10,8 @@ import '../widgets/pluri_icon.dart';
 import '../widgets/pluri_layout.dart';
 import '../widgets/pluri_premium_widgets.dart';
 import '../widgets/pluri_push_scaffold.dart';
+import '../widgets/pluri_root_header.dart';
+import '../widgets/pluri_sleep_timer_sheet.dart';
 import 'package:pluriwave/widgets/tarjeta_emisora.dart';
 import 'ajustes/pantalla_ajustes_emisoras_personalizadas.dart';
 import 'ajustes/pantalla_ajustes_grupos_favoritos.dart';
@@ -113,6 +115,14 @@ class _PantallaFavoritosState extends State<PantallaFavoritos> {
       return ListView(
         padding: PluriLayout.pageListPadding,
         children: [
+          // S1 (Tier 1 visual fidelity): the prototype has no global
+          // AppBar — this root now draws its own 56px title row instead of
+          // relying on app.dart's removed shared chrome (which is also
+          // where the sleep-timer action used to live).
+          PluriRootHeader(
+            title: l10n.favoritesTitle,
+            onSleepTimer: () => showPluriSleepTimerSheet(context),
+          ),
           PluriScreenHeader(
             title: l10n.favoritesTitle,
             subtitle: l10n.favoritesHeaderSubtitle,
@@ -180,6 +190,11 @@ class _PantallaFavoritosState extends State<PantallaFavoritos> {
         child: Column(
           crossAxisAlignment: CrossAxisAlignment.start,
           children: [
+            // S1 (Tier 1 visual fidelity): see the empty-state branch above.
+            PluriRootHeader(
+              title: l10n.favoritesTitle,
+              onSleepTimer: () => showPluriSleepTimerSheet(context),
+            ),
             PluriScreenHeader(
               title: l10n.favoritesTitle,
               subtitle: l10n.favoritesHeaderSubtitle,