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/.

test/estado/estado_busqueda_test.dart

@@ -1,5 +1,6 @@
 import 'package:flutter_test/flutter_test.dart';
 import 'package:pluriwave/estado/estado_busqueda.dart';
+import 'package:pluriwave/modelos/pais_radio.dart';
 
 import '../helpers/fakes.dart';
 
@@ -64,4 +65,61 @@ void main() {
       expect(identical(busqueda.resultados, busqueda.resultados), isTrue);
     },
   );
+
+  // ---------------------------------------------------------------------
+  // WU7, `station-discovery-browse` spec — Países browser over
+  // `ServicioRadio.obtenerPaises()`. In-memory cache guard: re-entering the
+  // Países screen must NOT re-fetch (design ADR-4).
+  // ---------------------------------------------------------------------
+  group('EstadoBusqueda — cargarPaises', () {
+    test(
+      'carga la lista de países y no vuelve a pedir en un 2º llamado',
+      () async {
+        final radio = FakeServicioRadio(
+          paises: const [
+            PaisRadio(nombre: 'Spain', codigoIso: 'ES', numeroEmisoras: 482),
+            PaisRadio(
+              nombre: 'Argentina',
+              codigoIso: 'AR',
+              numeroEmisoras: 120,
+            ),
+          ],
+        );
+        final busqueda = EstadoBusqueda(radio: radio);
+        addTearDown(busqueda.dispose);
+
+        expect(busqueda.paises, isEmpty);
+        expect(busqueda.cargandoPaises, isFalse);
+
+        await busqueda.cargarPaises();
+
+        expect(
+          busqueda.paises.map((p) => p.codigoIso),
+          containsAll(['ES', 'AR']),
+        );
+        expect(busqueda.cargandoPaises, isFalse);
+        expect(radio.obtenerPaisesCalls, 1);
+
+        await busqueda.cargarPaises();
+
+        // Cache guard: a 2nd call with data already present must not refetch.
+        expect(radio.obtenerPaisesCalls, 1);
+      },
+    );
+
+    test(
+      'falla con gracia: no deja cargandoPaises en true ni la lista poblada',
+      () async {
+        final busqueda = EstadoBusqueda(
+          radio: FakeServicioRadio(errorPaises: Exception('fallo de red')),
+        );
+        addTearDown(busqueda.dispose);
+
+        await busqueda.cargarPaises();
+
+        expect(busqueda.cargandoPaises, isFalse);
+        expect(busqueda.paises, isEmpty);
+      },
+    );
+  });
 }