fix(radio): discover live API mirrors instead of hardcoding dead ones

Two of the three Radio Browser hosts this client shipped no longer resolve.
The retry loop rotates de1 -> nl1 -> at1, so once the first attempt failed
for any transient reason the remaining two were guaranteed to fail as well:
the retries meant to add resilience had become a dead end, and a single blip
surfaced as "No connection to the radio API" with a healthy API and a healthy
network. The live mirror list confirms only one server remains:

  [{"ip":"91.98.4.78","name":"de1.api.radio-browser.info"},
   {"ip":"2a01:4f8:1c1d:699::1","name":"de1.api.radio-browser.info"}]

The API docs say exactly what this code was doing wrong: "Never use a direct
link to a single new server. It is much better to get a list of the servers",
pointing clients at all.api.radio-browser.info to enumerate what exists.

Seed with that round-robin host plus de1, then resolve the real list from
/json/servers once per instance and rotate over that. Discovery shares one
in-flight request across concurrent callers, because the home screen loads
two lists at once through Future.wait, and any failure silently leaves the
seed list in place — it still contains a working host, so a failed discovery
must never be worse than not trying. Explicitly injected servers disable
discovery so callers can still pin a mirror.

Build the User-Agent from the running package too. The API asks clients to
identify themselves, and this header claimed PluriWave/0.1.0 while the app
shipped 1.1.x. A literal cannot stay correct here — CI bumps the version on
every single release — so read it via package_info_plus, already a dependency
used in three other places. If package info is unavailable the product name
goes out alone rather than a made-up version, and resolution never throws: a
header must not be able to fail a request.

lib/servicios/servicio_radio.dart

@@ -1,6 +1,7 @@
 import 'dart:convert';
 
 import 'package:http/http.dart' as http;
+import 'package:package_info_plus/package_info_plus.dart';
 
 import '../modelos/emisora.dart';
 
@@ -9,15 +10,30 @@ import '../modelos/emisora.dart';
 /// Aplica reintentos acotados con rotación de host para tolerar fallos
 /// transitorios al iniciar.
 class ServicioRadio {
+  /// Product name for the `User-Agent` the API asks every client to send
+  /// ("Send a speaking http agent string"); requests without one may be
+  /// throttled.
+  static const _productoUserAgent = 'PluriWave';
+
   static const _timeoutPorDefecto = Duration(seconds: 10);
   static const _maxIntentosPorDefecto = 3;
   static const _retryDelayPorDefecto = Duration(milliseconds: 250);
 
-  // Servidores conocidos como fallback si el DNS falla.
-  static const _servidoresFallback = [
+  /// Bootstrap hosts, used only until the live mirror list is discovered.
+  ///
+  /// `all.api.radio-browser.info` is the round-robin entry point the API docs
+  /// point clients at; it tracks whichever mirrors exist without this app
+  /// shipping their names. `de1` follows as a concrete fallback for the case
+  /// where the round-robin record itself is unresolvable.
+  ///
+  /// Deliberately NOT a list of individual mirror names: the docs say "Never
+  /// use a direct link to a single new server. It is much better to get a list
+  /// of the servers", and the previous hardcoded `nl1`/`at1` entries proved the
+  /// point by being decommissioned — they stopped resolving, so both retries
+  /// after the first failure were guaranteed to fail too.
+  static const servidoresSemilla = [
+    'all.api.radio-browser.info',
     'de1.api.radio-browser.info',
-    'nl1.api.radio-browser.info',
-    'at1.api.radio-browser.info',
   ];
 
   ServicioRadio({
@@ -29,8 +45,11 @@ class ServicioRadio {
   }) : _cliente = cliente ?? http.Client(),
        _servidores =
            (servidores == null || servidores.isEmpty)
-               ? List<String>.from(_servidoresFallback)
+               ? List<String>.from(servidoresSemilla)
                : List<String>.from(servidores),
+       // Explicit servers mean the caller is pinning the list (tests, or a
+       // future user-configured mirror), so discovery must not override it.
+       _descubrimientoHecho = servidores != null && servidores.isNotEmpty,
        _maxIntentos = maxIntentos < 1 ? 1 : maxIntentos,
        _retryDelay = retryDelay,
        _timeout = timeout;
@@ -42,6 +61,31 @@ class ServicioRadio {
   final Duration _timeout;
 
   String? _servidorActual;
+  bool _descubrimientoHecho;
+  Future<void>? _descubrimientoEnCurso;
+  String? _userAgent;
+
+  /// Builds the `User-Agent` from the running build, once per instance.
+  ///
+  /// The version is read at runtime on purpose: CI bumps it on every release,
+  /// so a literal here goes stale immediately — this header claimed `0.1.0`
+  /// while the app shipped 1.1.x. When the package info is unavailable (unit
+  /// tests, any platform without the plugin) the product name goes out on its
+  /// own rather than a made-up version. Never throws: a header must not be able
+  /// to fail a request.
+  Future<String> _resolverUserAgent() async {
+    final cache = _userAgent;
+    if (cache != null) return cache;
+    try {
+      final info = await PackageInfo.fromPlatform();
+      final version = info.version.isNotEmpty ? '/${info.version}' : '';
+      final paquete =
+          info.packageName.isNotEmpty ? ' (${info.packageName})' : '';
+      return _userAgent = '$_productoUserAgent$version$paquete';
+    } catch (_) {
+      return _userAgent = _productoUserAgent;
+    }
+  }
 
   int _indiceServidorInicial() {
     if (_servidorActual == null) {
@@ -60,7 +104,61 @@ class ServicioRadio {
     return Uri.https(servidor, path, {'hidebroken': 'true', ...params});
   }
 
+  /// Replaces the seed list with the mirrors the API reports as live.
+  ///
+  /// Runs at most once per instance, and never blocks a request for long: any
+  /// failure leaves the seed list in place, which still contains a working
+  /// round-robin host. Concurrent callers share the same in-flight discovery
+  /// instead of each firing their own `/json/servers` request — the home screen
+  /// loads two lists at once through `Future.wait`.
+  Future<void> _descubrirServidores() {
+    if (_descubrimientoHecho) return Future<void>.value();
+    final enCurso = _descubrimientoEnCurso;
+    if (enCurso != null) return enCurso;
+
+    final descubrimiento = () async {
+      for (final semilla in _servidores.toList()) {
+        try {
+          final resp = await _cliente
+              .get(
+                Uri.https(semilla, '/json/servers'),
+                headers: {'User-Agent': await _resolverUserAgent()},
+              )
+              .timeout(_timeout);
+          if (resp.statusCode != 200) continue;
+
+          final lista = json.decode(resp.body) as List<dynamic>;
+          // One entry per IP family, so the same name repeats; keep insertion
+          // order and drop duplicates.
+          final nombres = <String>{};
+          for (final item in lista) {
+            if (item is! Map) continue;
+            final nombre = item['name'];
+            if (nombre is String && nombre.isNotEmpty) nombres.add(nombre);
+          }
+          if (nombres.isEmpty) continue;
+
+          _servidores
+            ..clear()
+            ..addAll(nombres);
+          _servidorActual = null;
+          return;
+        } on Exception {
+          // Try the next seed; the seed list stays usable either way.
+          continue;
+        }
+      }
+    }();
+
+    _descubrimientoEnCurso = descubrimiento.whenComplete(() {
+      _descubrimientoHecho = true;
+      _descubrimientoEnCurso = null;
+    });
+    return _descubrimientoEnCurso!;
+  }
+
   Future<List<Emisora>> _get(String path, Map<String, String> params) async {
+    await _descubrirServidores();
     Exception? ultimoError;
     final indiceBase = _indiceServidorInicial();
     final totalIntentos = _maxIntentos;
@@ -73,9 +171,7 @@ class ServicioRadio {
         final resp = await _cliente
             .get(
               uri,
-              headers: {
-                'User-Agent': 'PluriWave/0.1.0 (es.freetimelab.pluriwave)',
-              },
+              headers: {'User-Agent': await _resolverUserAgent()},
             )
             .timeout(_timeout);