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_inicio.dart

@@ -14,6 +14,8 @@ import '../tema/pluriwave_theme.dart';
 import '../widgets/pluri_glass_surface.dart';
 import '../widgets/pluri_layout.dart';
 import '../widgets/pluri_premium_widgets.dart';
+import '../widgets/pluri_root_header.dart';
+import '../widgets/pluri_sleep_timer_sheet.dart';
 import '../widgets/visualizador_audio.dart';
 import 'package:pluriwave/widgets/tarjeta_emisora.dart';
 
@@ -39,6 +41,16 @@ class _PantallaInicioState extends State<PantallaInicio> {
 
     return CustomScrollView(
       slivers: [
+        // 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).
+        SliverToBoxAdapter(
+          child: PluriRootHeader(
+            title: l10n.navHome,
+            onSleepTimer: () => showPluriSleepTimerSheet(context),
+          ),
+        ),
         // WU5 built the hero; WU6 relocated the discovery sections that
         // used to follow it (_seccionCercanas, _seccionTendencias,
         // _chipGeneros, _errorBanner, the browse grid) into
@@ -142,7 +154,6 @@ class _PantallaInicioState extends State<PantallaInicio> {
       ),
     );
   }
-
 }
 
 /// WU5, design ADR-7: the Escuchar embedded player. `EstadoRadio` is the