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).

test/app_test.dart

@@ -0,0 +1,23 @@
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+
+/// S1 (Tier 1 visual fidelity): the prototype (`t4`) never draws a global
+/// `AppBar` — every root owns its own 56px title row instead (see
+/// `PluriRootHeader` and `root_header_wiring_test.dart`). `app.dart` used to
+/// wrap every tab in `PluriWaveScaffold(appBar: AppBar(...))`; this is a
+/// fast source-level regression guard for that removal, since the widget
+/// under it (`_PaginaPrincipal`) is library-private and constructs real
+/// platform-backed services, so it cannot be safely widget-tested here.
+void main() {
+  test('app.dart no longer constructs a global Material AppBar', () {
+    final source = File('lib/app.dart').readAsStringSync();
+    expect(
+      source.contains('AppBar('),
+      isFalse,
+      reason:
+          'the prototype has no global app bar — each root draws its own '
+          'PluriRootHeader inside its content instead',
+    );
+  });
+}