feat(paises): add country browser and extract shared radio transport

Extracts ServicioRadio's transport loop (server discovery, host rotation,
bounded retries, User-Agent, timeout, status check, json.decode, sticky-host
bookkeeping) out of `_get` into a new `_getJson(path, params) ->
Future<List<dynamic>>` helper, moved as one block with no logic edits. `_get`
is reimplemented on top, still owning every station-specific concern:
`lastcheckok: '1'`, `Emisora.fromApi` + the empty-uuid/url filter, and the
`_compararCalidad` quality sort. `_getJson` is deliberately sort-agnostic and
filter-agnostic so a non-station endpoint can reuse the resilience behaviour
without inheriting station-only semantics.

Non-negotiable ordering followed per design ADR-4: new
test/servicios/servicio_radio_transporte_test.dart characterises all 8
existing station calls (7 via `_get` plus `registrarClick`, which builds its
own URI) against the UNMODIFIED `_get` first - green by construction -
pinning path, lastcheckok=1, hidebroken=true, a non-empty User-Agent, exact
order/reverse/limit/offset, and the exact returned UUID sequence from a
fixture with deliberately shuffled bitrate/clickcount/votes. That last
assertion is what makes the extraction safe: a sort that silently sank into
transport would pass every other check. Re-running the same file after the
extraction is byte-identical green. test/servicios/servicio_radio_test.dart
is untouched by this work unit - its passing unmodified is itself a signal
that transport wasn't disturbed.

The 6 pre-existing `order: bitrate` occurrences (obtenerPopulares,
buscarPorNombre, buscarPorPais, buscarPorIdioma, buscarPorTag, buscar) are
untouched - a deliberate server-side quality bias deciding which stations
return within `limit`, unrelated to and never to be confused with the
user-facing "Ordenar" control, which stays entirely client-side via the
existing OrdenEmisoras (Engram reference/radio-browser-sort-order).

Behaviour delta, accepted per ADR-4, not a regression: moving
`_servidorActual` bookkeeping into `_getJson` means a successful
`/json/countries` call now warms the sticky host for subsequent station
calls too - one shared warm mirror per instance, desirable, not per-call-type
state.

Adds the Paises browser over the verified `/json/countries` contract (Engram
reference/radio-browser-countries-endpoint): new lib/modelos/pais_radio.dart
(`PaisRadio.fromApi` parses `stationcount` via `int.tryParse` since the API
returns it as a JSON string, not an int - an `as int` cast would throw),
`obtenerPaises()` sends neither `lastcheckok` nor `order` (the screen sorts
client-side by name; the API's raw byte order isn't proper collation for any
locale this app ships), and inherits `hidebroken=true` from the unchanged
`_uri` (desirable here too, since the endpoint's own default is false).
`EstadoBusqueda` gains `paises`/`cargandoPaises`/`cargarPaises()` with an
in-memory cache guard so re-entering the screen never refetches.

New PantallaPaises (lib/pantallas/pantalla_paises.dart): a "Tus idiomas"
shortlist (one representative country per the app's 13 supported locales,
matched against the fetched list - the proposal/spec name this section but
don't specify its derivation) above the full alphabetical list, each entry
showing its parsed station count. Reachable from Buscar's discovery landing
state via a new entry row, added now rather than left dangling per this
file's own forward-reference comment (and the WU15/WU15b lesson: a
fully-tested but unreachable screen is a real defect, not a follow-up).

New ARB keys (en/es only, matching this change's established precedent):
countriesScreenTitle, countriesYourLanguagesTitle, countriesAllTitle,
radioCountriesError.

Tests: 631 -> 649 (2 skipped, unchanged). flutter analyze unchanged at 1
pre-existing info. grep confirms `countrycodes` appears nowhere in lib/.

lib/pantallas/pantalla_buscar.dart

@@ -11,8 +11,10 @@ import '../widgets/pluri_glass_surface.dart';
 import '../widgets/pluri_icon.dart';
 import '../widgets/pluri_layout.dart';
 import '../widgets/pluri_premium_widgets.dart';
+import '../widgets/pluri_push_scaffold.dart';
 import 'package:pluriwave/widgets/tarjeta_emisora.dart';
 
+import 'pantalla_paises.dart';
 import 'reproducir_minimizado.dart';
 
 const _paises = [
@@ -190,6 +192,7 @@ class _PantallaBuscarState extends State<PantallaBuscar> {
           _seccionCercanas(context, theme, l10n),
           _seccionTendencias(context, theme, l10n),
           _chipGeneros(context, theme, l10n),
+          _seccionPaises(context, theme, l10n),
           if (context.select<EstadoRadio, String?>((e) => e.error) != null)
             _errorBanner(
               context,
@@ -213,21 +216,15 @@ class _PantallaBuscarState extends State<PantallaBuscar> {
     final l10n = AppLocalizations.of(context);
     final pills = <Widget>[
       if (_paisSeleccionado != null)
-        _pillFiltro(
-          _paisLabelSeleccionado(l10n) ?? _paisSeleccionado!,
-          () {
-            setState(() => _paisSeleccionado = null);
-            _buscar();
-          },
-        ),
+        _pillFiltro(_paisLabelSeleccionado(l10n) ?? _paisSeleccionado!, () {
+          setState(() => _paisSeleccionado = null);
+          _buscar();
+        }),
       if (_idiomaSeleccionado != null)
-        _pillFiltro(
-          _idiomaLabelSeleccionado(l10n) ?? _idiomaSeleccionado!,
-          () {
-            setState(() => _idiomaSeleccionado = null);
-            _buscar();
-          },
-        ),
+        _pillFiltro(_idiomaLabelSeleccionado(l10n) ?? _idiomaSeleccionado!, () {
+          setState(() => _idiomaSeleccionado = null);
+          _buscar();
+        }),
       if (_calidadMinima != null)
         _pillFiltro('≥$_calidadMinima kbps', () {
           setState(() => _calidadMinima = null);
@@ -249,7 +246,8 @@ class _PantallaBuscarState extends State<PantallaBuscar> {
       child: Column(
         crossAxisAlignment: CrossAxisAlignment.start,
         children: [
-          if (pills.isNotEmpty) Wrap(spacing: 8, runSpacing: 8, children: pills),
+          if (pills.isNotEmpty)
+            Wrap(spacing: 8, runSpacing: 8, children: pills),
           if (pills.isNotEmpty && estado.resultados.isNotEmpty)
             const SizedBox(height: 10),
           if (!estado.cargando && estado.resultados.isNotEmpty)
@@ -438,9 +436,7 @@ class _PantallaBuscarState extends State<PantallaBuscar> {
                     selected: seleccionado == value,
                     visualDensity: VisualDensity.compact,
                     onSelected:
-                        (_) => onChanged(
-                          seleccionado == value ? null : value,
-                        ),
+                        (_) => onChanged(seleccionado == value ? null : value),
                   ),
               ],
             ),
@@ -486,9 +482,7 @@ class _PantallaBuscarState extends State<PantallaBuscar> {
                     selected: seleccionado == value,
                     visualDensity: VisualDensity.compact,
                     onSelected:
-                        (_) => onChanged(
-                          seleccionado == value ? null : value,
-                        ),
+                        (_) => onChanged(seleccionado == value ? null : value),
                   ),
               ],
             ),
@@ -527,7 +521,9 @@ class _PantallaBuscarState extends State<PantallaBuscar> {
             child: PluriEmptyState(
               glyph: PluriIconGlyph.search,
               title:
-                  sinFiltros ? l10n.searchEmptyTitle : l10n.searchNoResultsTitle,
+                  sinFiltros
+                      ? l10n.searchEmptyTitle
+                      : l10n.searchNoResultsTitle,
               subtitle:
                   sinFiltros
                       ? l10n.searchEmptySubtitle
@@ -803,6 +799,46 @@ class _PantallaBuscarState extends State<PantallaBuscar> {
     );
   }
 
+  /// WU7, `station-discovery-browse` spec: the "Países entry point" the
+  /// spec's landing-state scenario lists. Deferred from WU6 (this class's
+  /// own doc comment: `PantallaPaises` did not exist yet); added here now
+  /// that it does, alongside the screen it targets — an unreachable screen
+  /// would repeat the WU15/WU15b lesson (a fully-tested screen shipped with
+  /// no navigation path to it).
+  Widget _seccionPaises(
+    BuildContext context,
+    ThemeData theme,
+    AppLocalizations l10n,
+  ) {
+    return Padding(
+      padding: const EdgeInsets.fromLTRB(
+        PluriLayout.horizontal,
+        8,
+        PluriLayout.horizontal,
+        0,
+      ),
+      child: PluriGlassSurface(
+        padding: EdgeInsets.zero,
+        // `PluriGlassSurface` paints via `DecoratedBox`, not `Material` — a
+        // tappable `ListTile` needs its own `Material` ancestor or its ink
+        // splash silently fails to paint (Flutter's own debug assertion).
+        child: Material(
+          type: MaterialType.transparency,
+          child: ListTile(
+            leading: const Icon(Icons.public_rounded),
+            title: Text(l10n.countriesScreenTitle),
+            trailing: const Icon(Icons.chevron_right_rounded),
+            onTap:
+                () => PluriPushScaffold.push(
+                  context,
+                  (_) => const PantallaPaises(),
+                ),
+          ),
+        ),
+      ),
+    );
+  }
+
   Widget _errorBanner(
     BuildContext context,
     String error,
@@ -892,7 +928,11 @@ class _PantallaBuscarState extends State<PantallaBuscar> {
           (context, i) => TarjetaEmisora(
             emisora: emisoras[i],
             onTap: () => reproducirMinimizado(context, emisoras[i]),
-          ).pluriFadeSlideIn(context, delay: Duration(milliseconds: i * 30), beginY: 0.1),
+          ).pluriFadeSlideIn(
+            context,
+            delay: Duration(milliseconds: i * 30),
+            beginY: 0.1,
+          ),
     );
   }
 }