fix(grabacion): reject a local track as a recording source

Reported, with this on screen:

  No se pudo iniciar la grabación: Invalid argument(s): Unsupported scheme
  'content' in URI content://com.android.externalstorage.documents/tree/
  primary%3AMusic/document/primary%3AMusic%2F...%2FNew Limit - Smile.mp3

The URI in that message is a local MP3, not a station.
PluriWaveAudioHandler._cambiarFuente sets `emisoraActual` for EVERY source
it plays, so a local track surfaces as an Emisora whose `url` is the SAF
content:// document URI it was opened from. EstadoGrabacion.iniciar only
checked for null, handed that straight to the recorder, and the HTTP
client failed with a message no user can act on.

"It used to work" is exactly right: before local music playback existed,
whatever was playing was always a real station, so the case could not
arise. The recorder never changed.

iniciar() now also requires a real network stream (esEmisoraGrabable) and
falls back to the existing "select a station first" message, which is the
correct guidance here -- recording a local file makes no sense anyway,
it is already on the device. No new l10n key, so no 13-locale churn for a
message that already says the right thing.

Tests: 1158 -> 1161.

lib/estado/estado_grabacion.dart

@@ -18,6 +18,23 @@ import '../servicios/servicio_grabacion_radio.dart';
 /// `EstadoRadio` consumers (S4-R5). Playback orchestration (stop recording on
 /// pause/stop/station switch) stays in `EstadoRadio`, which keeps a reference
 /// to this notifier.
+/// Whether [emisora] is something the recorder can actually capture: a live
+/// network stream.
+///
+/// The recorder opens the URL as an HTTP stream and writes the bytes to disk,
+/// so anything else fails inside the HTTP client with a message no user can
+/// act on ("Unsupported scheme 'content' in URI content://...").
+///
+/// This is not hypothetical tidiness. `PluriWaveAudioHandler._cambiarFuente`
+/// sets `emisoraActual` for EVERY source it plays, so a local MP3 shows up
+/// here as an `Emisora` whose `url` is the `content://` document URI it was
+/// opened from. Recording a local file makes no sense anyway — it is already
+/// on the device.
+bool esEmisoraGrabable(Emisora emisora) {
+  final esquema = Uri.tryParse(emisora.url)?.scheme.toLowerCase();
+  return esquema == 'http' || esquema == 'https';
+}
+
 class EstadoGrabacion extends ChangeNotifier {
   EstadoGrabacion({
     ServicioGrabacionRadio? servicio,
@@ -72,7 +89,13 @@ class EstadoGrabacion extends ChangeNotifier {
 
   Future<void> iniciar({Duration? duracion}) async {
     final actual = _emisoraActual();
-    if (actual == null) {
+    // `emisoraActual` is set by `_cambiarFuente` for EVERY source, local
+    // tracks included -- a local file becomes an `Emisora` whose `url` is the
+    // SAF `content://` URI it was opened from. Handing that to the recorder
+    // produced "Unsupported scheme 'content' in URI content://..." on screen,
+    // and it started happening only once local music playback existed: before
+    // that, whatever was playing was always a real station.
+    if (actual == null || !esEmisoraGrabable(actual)) {
       _alError?.call(_textos.recordingSelectStationFirst);
       return;
     }