fix(auto): guard shipped resources, walk favourite groups when skipping

History review requested by the owner: when and why did the Android Auto
UI stop working.

ANSWER: 31 July, commit 2540556, "give the equalizer actions distinct,
state-aware icons".

  9eff760 (31-07)  androidIcon: 'drawable/ic_stat_pluriwave'  -> in the APK
  2540556 (31-07)  androidIcon: 'drawable/ic_auto_eq_on'      -> NEVER in it

That commit swapped a drawable that shipped for two that the stale CI
resource cache never included. From that moment getResourceId returned 0,
PlaybackStateCompat.CustomAction.Builder threw, and the throw aborted
AudioService.setState before the session was published -- so every Android
Auto symptom chased since is one line of that commit. The bitter part is
that 2540556 was itself a fix for a report about two identical icons.

Three changes.

1. CI guard. The build now unzips the release APK and fails if a drawable
resolved by NAME at runtime is missing. Resolution by name cannot fail at
compile time -- it fails in the car, silently, with id 0. This class of
bug shipped undetected for a week; it cannot ship again.

2. Skipping stations now walks the favourites GROUP first, as requested:
group -> all favourites -> my stations -> catalogue. Two deliberate
exclusions, both tested: `sinAsignarId` is the ABSENCE of a group, not a
group, so those walk all favourites; and a one-member group falls through
too, or both buttons would be dead ends. The group is read from the
FAVOURITE record, never from the playing station -- that one is rebuilt by
emisoraDesdeMediaItem, which carries no group id and would always report
"unfiled".

3. Diagnostics on the station skip. It was reported as doing nothing for
radio, and every early return in that method is silent: an empty list and
a single-entry list look identical from outside. The log now names which
one fired, so the next capture answers it instead of another hypothesis.

Tests: 1161 -> 1165.

lib/servicios/servicio_audio.dart

@@ -1544,6 +1544,15 @@ class PluriWaveAudioHandler extends BaseAudioHandler
         todas: await fuente.todas(),
       );
       final destino = emisoraVecina(actual, lista, haciaAtras: haciaAtras);
+      // Reported: in the car these buttons did nothing for radio. Every early
+      // return here is silent, so the log has to say WHICH one fired --
+      // an empty list (the station matched none of the three) and a station
+      // that is in a list of one are indistinguishable from outside.
+      debugPrint(
+        '[PluriWave][ServicioAudio] salto emisora atras=$haciaAtras '
+        'actual=${actual.nombre} uuid=${actual.uuid} '
+        'lista=${lista.length} destino=${destino?.nombre ?? "NINGUNO"}',
+      );
       if (destino == null) return;
       await playMediaItem(mediaItemParaEmisora(destino, l10n: _textos));
     } catch (e) {