fix(auto): fall back to on-brand artwork when a station or track has none

Stations and tracks with no artwork showed empty tiles in the car. The
browse tree's itemEmisora/_itemLocal already fell back to the rotating
station_art_* drawable via artUriPara/artUriLocal, but the "now playing"
MediaItem built when actually playing something (car tap, phone-initiated
play, folder-queue advance, direct local-track tap) did not, so the car's
now-playing screen still went blank.

Reuse the SAME artUriPara/artUriLocal fallback (already the project's one
selection scheme, mirroring PluriStationArtFallback) at every "now playing"
construction site: reproducirPorMediaId, ServicioAudio.reproducir (now via
the extracted, unit-tested mediaItemParaEmisora), construirMediaItemColaLocal
and reproducirPistaLocal.

Guard the reverse direction too: emisoraDesdeMediaItem (extracted from the
handler's private method, now unit-tested) only reflects artUri back into
Emisora.favicon when it passes faviconUsable, so the phone UI's
CachedNetworkImage widgets never attempt a doomed fetch of the car's
android.resource:// fallback URI -- they keep falling back to
PluriStationArtFallback exactly as before.

lib/servicios/navegacion_auto.dart

@@ -836,10 +836,12 @@ Future<void> reproducirPorMediaId(
     title: emisora.nombre,
     artist: emisora.pais ?? '',
     album: 'PluriWave',
-    artUri:
-        emisora.favicon != null && emisora.favicon!.isNotEmpty
-            ? Uri.tryParse(emisora.favicon!)
-            : null,
+    // Item 3: reuses [artUriPara] (the SAME fallback the browse tree's
+    // itemEmisora already applies) so the "now playing" media item never
+    // falls back to a blank tile — a real usable favicon still wins, a
+    // missing/unusable one gets the on-brand rotating drawable instead of
+    // `null`.
+    artUri: Uri.parse(artUriPara(emisora)),
     extras: {'uuid': emisora.uuid},
   );
   await reproducir(item);
@@ -1143,6 +1145,11 @@ Future<MediaItem?> construirMediaItemColaLocal(
     id: contentUri,
     title: _tituloDesdeDocumentId(nodo.documentId),
     album: 'PluriWave',
+    // Item 3: a queued local track had NO artUri at all before — reuses
+    // [artUriLocal] (the SAME on-brand rotation the browse tree's
+    // `_itemLocal` already falls back to) so the car's now-playing screen
+    // never shows a blank tile for a track with no embedded art.
+    artUri: Uri.parse(artUriLocal(nodo.documentId)),
     extras: {'documentId': nodo.documentId},
   );
 }
@@ -1322,6 +1329,9 @@ Future<void> reproducirPistaLocal(
     id: pista.contentUri,
     title: pista.titulo,
     album: 'PluriWave',
+    // Item 3: same fallback as construirMediaItemColaLocal, for a track
+    // tapped directly (not via a folder-play queue).
+    artUri: Uri.parse(artUriLocal(pista.documentId)),
     extras: {'documentId': pista.documentId},
   );
   await reproducir(item);

lib/servicios/servicio_audio.dart

@@ -60,6 +60,46 @@ void registrarFuenteMusicaLocal(FuenteMusicaLocalAuto fuente) {
   _fuenteMusicaLocalGlobal = fuente;
 }
 
+/// Builds the phone-initiated "play a station" `MediaItem` (item 3, Android
+/// Auto fallback artwork): reuses [artUriPara] (`navegacion_auto.dart`) so a
+/// station with no usable favicon gets the SAME on-brand rotating fallback
+/// the browse tree and the car-tap path already show, instead of a blank
+/// tile on the car/lockscreen/notification. Pure — no [PluriWaveAudioHandler]
+/// dependency — so it is unit-testable without instantiating the handler.
+MediaItem mediaItemParaEmisora(Emisora emisora, {required AppLocalizations l10n}) {
+  return MediaItem(
+    id: emisora.url,
+    title: localizedStationName(l10n, emisora.nombre),
+    artist: emisora.pais ?? '',
+    album: 'PluriWave',
+    artUri: Uri.parse(artUriPara(emisora)),
+    extras: {'uuid': emisora.uuid},
+  );
+}
+
+/// Reconstructs the phone-side [Emisora] from the handler's current
+/// [MediaItem] (item 3): gates `favicon` through [faviconUsable]
+/// (`navegacion_auto.dart`) so a car/car-tap "now playing" item's on-brand
+/// FALLBACK `artUri` (an `android.resource://` drawable, never a real
+/// favicon) is never misread as a genuine station favicon — the phone UI's
+/// `CachedNetworkImage` widgets gate only on `favicon != null && isNotEmpty`
+/// (not on `faviconUsable`'s scheme check), so without this guard they would
+/// attempt a doomed network fetch of the fallback's non-http URI before
+/// falling back to [PluriStationArtFallback] themselves. A genuine http(s)
+/// favicon still round-trips exactly as before. Pure — no handler
+/// dependency — unit-testable directly.
+Emisora emisoraDesdeMediaItem(MediaItem mediaItem) {
+  final uuid = mediaItem.extras?['uuid'] as String? ?? mediaItem.id;
+  final artUriTexto = mediaItem.artUri?.toString();
+  return Emisora(
+    uuid: uuid,
+    nombre: mediaItem.title,
+    url: mediaItem.id,
+    pais: (mediaItem.artist?.isNotEmpty ?? false) ? mediaItem.artist : null,
+    favicon: faviconUsable(artUriTexto) ? artUriTexto : null,
+  );
+}
+
 /// Wrapper de alto nivel para el UI.
 class ServicioAudio {
   PluriWaveAudioHandler get _handler {
@@ -94,19 +134,9 @@ class ServicioAudio {
       });
 
   Future<void> reproducir(Emisora emisora) async {
-    final item = MediaItem(
-      id: emisora.url,
-      title: localizedStationName(
-        lookupAppLocalizations(const Locale('es')),
-        emisora.nombre,
-      ),
-      artist: emisora.pais ?? '',
-      album: 'PluriWave',
-      artUri:
-          emisora.favicon != null && emisora.favicon!.isNotEmpty
-              ? Uri.tryParse(emisora.favicon!)
-              : null,
-      extras: {'uuid': emisora.uuid},
+    final item = mediaItemParaEmisora(
+      emisora,
+      l10n: lookupAppLocalizations(const Locale('es')),
     );
     await _handler.playMediaItem(item);
   }
@@ -911,14 +941,10 @@ class PluriWaveAudioHandler extends BaseAudioHandler
   }
 
   Emisora _emisoraDesdeMediaItem(MediaItem mediaItem) {
-    final uuid = mediaItem.extras?['uuid'] as String? ?? mediaItem.id;
-    return Emisora(
-      uuid: uuid,
-      nombre: mediaItem.title,
-      url: mediaItem.id,
-      pais: (mediaItem.artist?.isNotEmpty ?? false) ? mediaItem.artist : null,
-      favicon: mediaItem.artUri?.toString(),
-    );
+    // Item 3: delegates to the top-level, unit-testable function so the
+    // `faviconUsable` guard (never reflect the on-brand fallback artUri
+    // back as a real favicon) is covered without instantiating the handler.
+    return emisoraDesdeMediaItem(mediaItem);
   }
 
   // ── Android Auto browsing (thin delegation to navegacion_auto.dart's