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/navegacion_auto.dart

@@ -936,6 +936,59 @@ Future<void> reproducirPorMediaId(
   await reproducir(item);
 }
 
+/// Picks the station a spoken query refers to ("pon Radio Clásica"), over the
+/// stations the car can already browse.
+///
+/// Pure and source-agnostic so it is testable without a handler. Ranking, best
+/// first:
+///   1. exact name match (case/accent-insensitive),
+///   2. name starts with the query,
+///   3. name contains the query,
+///   4. country contains the query.
+/// Ties are broken by the order [candidatas] arrives in, which the caller
+/// composes as favourites → my stations → all, so a favourite always wins over
+/// a stranger with the same name.
+///
+/// Returns `null` for an empty query or no match — the caller must then do
+/// nothing rather than play something arbitrary, since a driver who asked for
+/// a specific station is worse served by a random one than by silence.
+Emisora? emisoraParaBusqueda(String consulta, List<Emisora> candidatas) {
+  final q = _normalizarBusqueda(consulta);
+  if (q.isEmpty) return null;
+
+  Emisora? contiene;
+  Emisora? empieza;
+  Emisora? porPais;
+  for (final emisora in candidatas) {
+    final nombre = _normalizarBusqueda(emisora.nombre);
+    if (nombre == q) return emisora;
+    if (empieza == null && nombre.startsWith(q)) {
+      empieza = emisora;
+    } else if (contiene == null && nombre.contains(q)) {
+      contiene = emisora;
+    } else if (porPais == null &&
+        _normalizarBusqueda(emisora.pais ?? '').contains(q)) {
+      porPais = emisora;
+    }
+  }
+  return empieza ?? contiene ?? porPais;
+}
+
+/// Lowercase, accent-stripped, collapsed whitespace — a driver saying "radio
+/// clasica" must match "Radio Clásica", and voice transcription rarely gets
+/// diacritics right.
+String _normalizarBusqueda(String texto) {
+  const conAcento = 'áàäâãéèëêíìïîóòöôõúùüûñç';
+  const sinAcento = 'aaaaaeeeeiiiiooooouuuunc';
+  final buffer = StringBuffer();
+  for (final rune in texto.toLowerCase().runes) {
+    final char = String.fromCharCode(rune);
+    final i = conAcento.indexOf(char);
+    buffer.write(i >= 0 ? sinAcento[i] : char);
+  }
+  return buffer.toString().replaceAll(RegExp(r'\s+'), ' ').trim();
+}
+
 /// Routing seam for a car-tapped `eq_preset:<...>` media id (decision
 /// `auto/ecualizador-diseno`, mirrors [reproducirPorMediaId]'s seam
 /// shape): dispatches "Desactivar" to [activarEcualizador]`(false)`, and a