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_buscar.dart

@@ -12,6 +12,8 @@ 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 'pantalla_paises.dart';
@@ -140,6 +142,14 @@ class _PantallaBuscarState extends State<PantallaBuscar> {
     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.searchScreenTitle,
+          onSleepTimer: () => showPluriSleepTimerSheet(context),
+        ),
         PluriScreenHeader(
           title: l10n.searchScreenTitle,
           subtitle: l10n.searchScreenSubtitle,