fix(auto): advertise the transport actions Android for Cars requires

Reported: on the Android Auto playback screen the play/pause button stays
on PLAY while audio is audibly playing, and "it used to work, in the
latest versions it doesn't".

Previous rounds looked for a regression in this repo's audio commits and
found none: every playbackState.add site publishes playing:true with a
ready processingState, and AudioService.getPlaybackState maps that to
STATE_PLAYING. That search was aimed at the wrong thing.

The Android for Cars guide ("Enable playback control") is explicit:
"Android Auto and AAOS display playback controls based on the actions
that are enabled in the PlaybackStateCompat object. By default, your app
must support the following actions: ACTION_PLAY, ACTION_PAUSE,
ACTION_STOP, ACTION_PLAY_FROM_MEDIA_ID, ACTION_PLAY_FROM_SEARCH."

systemActions has carried only `seek` + `stop` since e9d1f67, the first
commit of the project -- git log -S confirms it was never once edited. So
the required actions have never been advertised, and no audio commit can
explain a change in behaviour. Android Auto ships as its own app and
updates itself, which is how a working screen breaks with a clean repo
history. That fits the report better than any commit here does.

The phone notification was never affected: it builds its play/pause
button from `controls`, not from these bits, which is exactly why the
symptom is car-only.

Skip actions stay conditional on an active queue on purpose -- the same
guide notes Auto reserves the prev/next slots for them and gives the
space to custom actions when the app does not support them, and that is
the space the equalizer toggle needs.

ACTION_PLAY_FROM_SEARCH is now implemented rather than merely claimed:
advertising it unimplemented would have the car's assistant accept "play
Radio X" and silently do nothing. emisoraParaBusqueda ranks exact name,
then prefix, then substring, then country, accent- and case-insensitive
because voice transcription rarely gets diacritics right; favourites are
searched first so they win a name tie, and a miss plays nothing rather
than something arbitrary.

Still a hypothesis for the play/pause symptom, not a confirmed fix -- it
is documentation-backed and cheap, but only a head unit can confirm it.

Tests: 1132 -> 1141.

lib/servicios/servicio_audio.dart

@@ -709,9 +709,32 @@ class PluriWaveAudioHandler extends BaseAudioHandler
             colaActiva: colaActiva,
             playing: playing,
           ),
+          // Android for Cars, "Enable playback control": «Android Auto and
+          // AAOS display playback controls based on the actions that are
+          // enabled in the PlaybackStateCompat object. By default, your app
+          // must support the following actions: ACTION_PLAY, ACTION_PAUSE,
+          // ACTION_STOP, ACTION_PLAY_FROM_MEDIA_ID, ACTION_PLAY_FROM_SEARCH.»
+          //
+          // This set had carried only `seek` + `stop` since the very first
+          // commit, so the required transport actions were never advertised.
+          // The car got away with it for a long time — but Android Auto is a
+          // separate app that updates itself, so a tolerance it used to have
+          // can disappear without a single line changing on our side. That
+          // matches the report exactly: "it used to work, and in the latest
+          // versions it doesn't", with no audio commit in between that could
+          // explain it.
+          //
+          // The phone notification never depended on any of this: it builds
+          // its play/pause button from `controls`, which is why the symptom
+          // is car-only.
           systemActions: {
-            MediaAction.seek,
+            MediaAction.play,
+            MediaAction.pause,
+            MediaAction.playPause,
             MediaAction.stop,
+            MediaAction.playFromMediaId,
+            MediaAction.playFromSearch,
+            MediaAction.seek,
             if (colaActiva) MediaAction.skipToPrevious,
             if (colaActiva) MediaAction.skipToNext,
           },
@@ -1622,6 +1645,42 @@ class PluriWaveAudioHandler extends BaseAudioHandler
     }
   }
 
+  /// Voice search from the car ("pon Radio Clásica").
+  ///
+  /// `ACTION_PLAY_FROM_SEARCH` is one of the actions Android for Cars
+  /// documents as required, and it is now advertised in `systemActions` — so
+  /// it has to actually do something. Advertising it unimplemented would be
+  /// worse than omitting it: the assistant would accept the command and
+  /// nothing would play, with no error to explain it.
+  ///
+  /// Favourites first, then my stations, then the full list, so a station the
+  /// driver already cares about wins a name tie. Never throws and never plays
+  /// something arbitrary on a miss — see [emisoraParaBusqueda].
+  @override
+  Future<void> playFromSearch(
+    String query, [
+    Map<String, dynamic>? extras,
+  ]) async {
+    try {
+      final fuente = _fuenteNavegacionGlobal;
+      if (fuente == null) return;
+      final candidatas = <Emisora>[
+        ...await fuente.favoritos(),
+        ...await fuente.misEmisoras(),
+        ...await fuente.todas(),
+      ];
+      final emisora = emisoraParaBusqueda(query, candidatas);
+      if (emisora == null) return;
+      await playMediaItem(mediaItemParaEmisora(emisora, l10n: _textos));
+    } catch (e) {
+      developer.log(
+        '[PluriWave] Error en playFromSearch($query): $e',
+        name: 'ServicioAudio',
+        level: 900,
+      );
+    }
+  }
+
   @override
   Future<void> playFromMediaId(
     String mediaId, [