fix(auto): keep the service alive through interruptions, restore local music

Four car reports, two root causes.

1. Local music vanished from the Android Auto menu. Self-inflicted, by
c1afe72 yesterday.

That commit moved registrarFuenteNavegacion above every await to keep a
headless engine from dying before it ran -- but left
registrarFuenteMusicaLocal below `await SharedPreferences.getInstance()`.
The root menu decides whether to offer "Música Local" with
`fuenteLocal != null && await fuenteLocal.hayCarpetaConfigurada()`, so
the car could now get a root response in the window between the two
registrations, find a null source, and be told there is no local music.
Android Auto caches the browse root, so it stayed missing for the whole
session. Before the reorder both registrations sat together after the
await and the window did not exist.

FuenteMusicaLocalAutoImpl never needed prefs to be CONSTRUCTED -- it
resolves them lazily per call, the same convention ServicioAlarmas uses
-- so it now registers beside the station source, above every await, and
the window is gone rather than narrowed.

2. PluriWave disappeared from the Auto pane mid-drive, the playback
screen sat frozen, and the equalizer was lost on every navigation
prompt. One cause for all three.

androidWillPauseWhenDucked: true made audio_session translate Android's
AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK into a full PAUSE. In a car that
fires constantly: every navigation instruction, every speed-camera
warning, every voice assistant. And a pause publishes playing:false,
which AudioService.setState turns into exitPlayingState() and, with
androidStopForegroundOnPause: true, into stopForeground(...). The
plugin's own doc for that flag says what follows: "while in this lower
priority state, the operating system will also be able to kill your
service at any time to reclaim resources". A killed service is a media
session that vanishes from the car pane -- and another media app takes
the slot.

Now the app ducks instead of pausing, so playing stays true and session,
notification and pane all survive an interruption; and the service stays
foreground even on a real pause, so a genuine one is not a death
sentence either. androidNotificationOngoing goes to false because the
plugin asserts it implies stopForegroundOnPause, and nothing is lost: a
foreground service already forces the notification to be ongoing.

A real, non-duckable focus loss (a phone call) still pauses and still
auto-resumes -- asserted, so the duck change cannot silently turn a call
into a station playing over it.

3. Previous/next on the car playback screen, for stations too.

skipToPrevious/skipToNext are now advertised unconditionally, since
Android Auto only draws those buttons when the app declares support.
They are no longer inert without a local queue: they walk the narrowest
list the current station belongs to -- favourites, then my stations,
then the catalogue -- wrapping at both ends, because a button that goes
dead at the end of a list reads as broken on a screen with no visible
list position. Matching is by uuid so a refreshed snapshot still
resolves, and a station in no list leaves playback untouched.

The equalizer toggle still fits alongside them: prev/next take their two
reserved slots and the equalizer claims the remaining custom-action room
because construirControlesTransporte places it before MediaControl.stop.

The phone notification is deliberately untouched: `controls` still gates
skip on an active queue, so nativeActions and
androidCompactActionIndices are byte-identical. Only systemActions
changed, and only the car reads those.

Tests: 1146 -> 1158.

lib/main.dart

@@ -26,8 +26,30 @@ const androidNotificationIconResource = 'drawable/ic_stat_pluriwave';
 const configuracionAudioService = AudioServiceConfig(
   androidNotificationChannelId: 'es.freetimelab.pluriwave.audio',
   androidNotificationChannelName: 'PluriWave Radio',
-  androidNotificationOngoing: true,
-  androidStopForegroundOnPause: true,
+  // Paired with `androidStopForegroundOnPause: false` below, and required to
+  // be: the plugin asserts `androidNotificationOngoing` implies
+  // `androidStopForegroundOnPause`. Nothing is lost by turning it off —
+  // while the service is in the foreground the OS forces the notification to
+  // be ongoing anyway, which is now the whole time playback is alive.
+  androidNotificationOngoing: false,
+  // The service stays in the FOREGROUND while paused.
+  //
+  // With `true`, a pause called `stopForeground(...)`, and a service that is
+  // not in the foreground is a service Android may kill at will. In the car
+  // that is exactly what happened: an interruption paused playback, the
+  // service dropped out of the foreground, Android reclaimed it, and
+  // PluriWave disappeared from the Android Auto pane — another media app
+  // took the slot. Ducking (see `ServicioAudioSession.configurar`) removes
+  // most pauses, but a real pause must not be a death sentence either.
+  //
+  // The plugin's own doc for this flag says it outright: «while in this
+  // lower priority state, the operating system will also be able to kill
+  // your service at any time to reclaim resources».
+  //
+  // Cost of `false`: the notification is not swipe-dismissible while paused,
+  // only after Stop. That is how every serious media app behaves, and Stop
+  // still tears everything down.
+  androidStopForegroundOnPause: false,
   notificationColor: PluriWaveTokens.brand,
   androidNotificationIcon: androidNotificationIconResource,
 );
@@ -56,6 +78,26 @@ Future<void> main() async {
   final fuenteAuto = FuenteEmisorasAutoLocal();
   registrarFuenteNavegacion(fuenteAuto);
 
+  // Local music registers HERE, above every await, alongside the station
+  // source — not after `SharedPreferences.getInstance()` where it used to
+  // sit.
+  //
+  // Regression this fixes, self-inflicted by the reordering above: the root
+  // menu decides whether to offer "Música Local" with
+  // `fuenteLocal != null && await fuenteLocal.hayCarpetaConfigurada()`.
+  // Moving ONLY the station source above the awaits meant the car could get
+  // a root response in the window before this line ran, find a null source,
+  // and be told there is no local music — and Android Auto caches the browse
+  // root, so it stayed missing for the whole session. Before the reorder
+  // both registrations sat together after the await, so the window did not
+  // exist.
+  //
+  // `FuenteMusicaLocalAutoImpl` needs no prefs to be CONSTRUCTED: it
+  // resolves them lazily per call (`_resolverPrefs`, falling back to
+  // `getInstance()`), the same convention `ServicioAlarmas` uses. So there
+  // was never a reason for it to wait on that await.
+  registrarFuenteMusicaLocal(FuenteMusicaLocalAutoImpl());
+
   // Cosmetic, and deliberately NOT awaited: a display preference must never
   // gate `runApp`. `_OrientacionResponsiveApp.didChangeDependencies` applies
   // it again as soon as a real view exists, which is the only moment it can
@@ -66,13 +108,6 @@ Future<void> main() async {
   // injected into every state/service below.
   final prefs = await SharedPreferences.getInstance();
 
-  // Local-music browse source (Design "getChildren data source
-  // registration"), same injectable-prefs DI convention as every other
-  // startup service — required so `_fuenteMusicaLocalGlobal` is ever
-  // non-null; without this registration the local-music root would stay
-  // permanently hidden (`hayCarpetaConfigurada()` has no fuente to ask).
-  registrarFuenteMusicaLocal(FuenteMusicaLocalAutoImpl(prefs: prefs));
-
   // User-saved EQ presets for the car's Ecualizador folder, same
   // injectable-prefs DI convention and same pre-init placement as the two
   // registrations above (neither depends on the AudioHandler). Passed as a