fix(chrome): rebuild MiniReproductor as a full-bleed opaque bar with art

Item 22 / audit 3.6 (t4:184-188): the mini player was a floating
999-radius glass pill with no artwork. Replace it with a 60px opaque
bar (listSurface at .97 alpha), full-bleed edge to edge, showing the
station's square 42x42 artwork instead of the abstract playing-bars
indicator. app.dart no longer wraps the bar in the balloon nav's own
8px side margin, so it now spans the full width.

MiniReproductor.altura is re-measured (72 -> 60) now that the bar's
content height is fixed by construction; PluriLayout.bottomChromeInset
derives from it as before. Both the S3-R3 configurarLocalizaciones
guard and the altura measurement test still pass unmodified.

lib/app.dart

@@ -243,23 +243,26 @@ class _PaginaPrincipalState extends State<_PaginaPrincipal>
       bottomNavigationBar: SafeArea(
         top: false,
         minimum: const EdgeInsets.only(bottom: PluriLayout.compactGap),
-        child: Padding(
-          padding: const EdgeInsets.fromLTRB(8, 0, 8, 0),
-          child: Column(
-            mainAxisSize: MainAxisSize.min,
-            children: [
-              // ADR-7(b): hidden on Escuchar (index 0) only — its embedded
-              // hero already shows the same station. Stays mounted (visible:
-              // false renders SizedBox.shrink(), not tree removal) so its
-              // didChangeDependencies side effect (S3-R3) keeps running.
-              MiniReproductor(visible: indice != RaizPluriWave.escuchar.index),
-              PluriBottomNavigation(
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            // Item 22 / audit 3.6 (t4:185 left:0;right:0): the mini player
+            // is full-bleed — it does NOT share the balloon bar's 8px side
+            // margin below. ADR-7(b): hidden on Escuchar (index 0) only —
+            // its embedded hero already shows the same station. Stays
+            // mounted (visible: false renders SizedBox.shrink(), not tree
+            // removal) so its didChangeDependencies side effect (S3-R3)
+            // keeps running.
+            MiniReproductor(visible: indice != RaizPluriWave.escuchar.index),
+            Padding(
+              padding: const EdgeInsets.fromLTRB(8, 0, 8, 0),
+              child: PluriBottomNavigation(
                 items: _navItems(l10n),
                 selectedIndex: indice,
                 onSelected: (i) => navegacion.irA(RaizPluriWave.values[i]),
               ),
-            ],
-          ),
+            ),
+          ],
         ),
       ),
     );