feat: confirm before leaving a game, and fix untranslated strings

Exit confirmation
- A dialog in the app's own style now guards every in-game screen. The safe
  action is the prominent one, so a stray tap keeps you in the game.
- Eleven screens covered, against both the system back gesture and the close
  buttons. The message adapts: a single-device game is lost, a host ends it
  for everyone, a client can rejoin.
- The two screens the host also opens from the manager take a flag so that
  back there just closes them instead of offering to quit.

Translations
- 21 keys were still holding the English text in Arabic, Basque, Hindi,
  Japanese, Korean, Dutch, Polish, Russian, Turkish and both Chinese
  variants: 231 strings, covering the whole multi-device flow, permissions
  and the mode picker. Nothing failed, the app just showed English.
- The permissions dialog was hardcoded in Spanish; it is localized now.
- Portuguese used Spanish gerunds where pt-PT takes "a + infinitive".

Checked what looked untranslated but is not: Animals and Notes are correct
Catalan, Configuration and Vibration correct French, Version correct German.
Those are whitelisted in the new test rather than silently rewritten.

l10n_cobertura_test guards all of it: no missing or orphan keys, no empty
values, no English left outside the whitelist, and placeholders preserved.

test/l10n_cobertura_test.dart

@@ -0,0 +1,110 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+
+/// Guardas sobre los ficheros de traducción.
+///
+/// El fallo que motivó estos tests: 21 claves se quedaron con el texto inglés
+/// dentro de árabe, japonés, coreano, chino, ruso, turco, hindi, neerlandés,
+/// polaco y euskera. Nada fallaba: la app mostraba inglés y ya está.
+void main() {
+  const idiomaPlantilla = 'es';
+
+  /// Coincidencias reales entre idiomas: palabras que se escriben igual y por
+  /// tanto no son un descuido. Cada entrada es "idioma:clave".
+  const coincidenciasLegitimas = {
+    // Nombre de la app y rangos numéricos, iguales en todas partes.
+    'de:appTitle', 'en:appTitle', 'fr:appTitle', 'it:appTitle',
+    // "Animals", "Professions", "Impostors", "Notes" son catalán correcto.
+    'ca:categoryAnimals', 'ca:categoryProfessions', 'ca:impostors',
+    'ca:notes', 'ca:notesTitle',
+    // "Vibration", "Version", "Name" son alemán correcto.
+    'de:vibration', 'de:version', 'de:profileName',
+    // "Sports", "Configuration", "Notes", "Vibration", "Version" son francés.
+    'fr:categorySports', 'fr:configuration', 'fr:notes', 'fr:notesTitle',
+    'fr:vibration', 'fr:version',
+    // "Round" se usa tal cual en italiano.
+    'it:roundNumber', 'it:roundElimination',
+  };
+
+  Map<String, String> cargar(String idioma) {
+    final fichero = File('lib/l10n/app_$idioma.arb');
+    final datos = json.decode(fichero.readAsStringSync()) as Map<String, dynamic>;
+    return {
+      for (final e in datos.entries)
+        if (!e.key.startsWith('@') && e.value is String)
+          e.key: e.value as String,
+    };
+  }
+
+  final idiomas = Directory('lib/l10n')
+      .listSync()
+      .whereType<File>()
+      .where((f) => f.path.endsWith('.arb'))
+      .map((f) => f.uri.pathSegments.last.replaceAll(RegExp(r'^app_|\.arb$'), ''))
+      .toList()
+    ..sort();
+
+  final plantilla = cargar(idiomaPlantilla);
+  final ingles = cargar('en');
+
+  test('están los 18 idiomas soportados', () {
+    expect(idiomas.length, 18);
+  });
+
+  for (final idioma in idiomas) {
+    group('app_$idioma.arb', () {
+      final valores = cargar(idioma);
+
+      test('no le falta ninguna clave de la plantilla', () {
+        final faltan = plantilla.keys.where((k) => !valores.containsKey(k));
+        expect(faltan, isEmpty, reason: 'sin traducir: ${faltan.join(", ")}');
+      });
+
+      test('no tiene claves que la plantilla no conozca', () {
+        final sobran = valores.keys.where((k) => !plantilla.containsKey(k));
+        expect(sobran, isEmpty, reason: 'huérfanas: ${sobran.join(", ")}');
+      });
+
+      test('ningún valor está vacío', () {
+        final vacios = valores.entries
+            .where((e) => e.value.trim().isEmpty)
+            .map((e) => e.key);
+        expect(vacios, isEmpty);
+      });
+
+      if (idioma != 'en' && idioma != idiomaPlantilla) {
+        test('no se ha quedado el texto en inglés', () {
+          final enIngles = valores.entries
+              .where((e) =>
+                  ingles[e.key] == e.value &&
+                  plantilla[e.key] != e.value &&
+                  !coincidenciasLegitimas.contains('$idioma:${e.key}'))
+              .map((e) => '${e.key} = "${e.value}"')
+              .toList();
+          expect(enIngles, isEmpty, reason: enIngles.join('\n'));
+        });
+      }
+
+      test('conserva los marcadores de posición de la plantilla', () {
+        final patron = RegExp(r'\{(\w+)\}');
+        final fallos = <String>[];
+        for (final entrada in valores.entries) {
+          final esperados = patron
+              .allMatches(plantilla[entrada.key] ?? '')
+              .map((m) => m.group(1)!)
+              .toSet();
+          final presentes =
+              patron.allMatches(entrada.value).map((m) => m.group(1)!).toSet();
+          // Los plurales ICU declaran el contador fuera de las llaves simples.
+          if (entrada.value.contains(', plural,')) continue;
+          if (!presentes.containsAll(esperados)) {
+            fallos.add('${entrada.key}: faltan ${esperados.difference(presentes)}');
+          }
+        }
+        expect(fallos, isEmpty, reason: fallos.join('\n'));
+      });
+    });
+  }
+}