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);