feat: discovery automático + QR como fallback en PantallaUnirse

- Discovery: busca hosts cercanos automáticamente y los muestra en lista
- Cada host aparece como tile tocable con nombre de la sala
- QR fallback: botón 'Escanear QR' debajo de la lista
- ServicioNearby: hostsEncontrados map, pararBusqueda(), no auto-connect
- Flujo: nombre → buscar → lista de salas (o QR) → conectar → espera
- l10n: searchGames, searchingGames, noGamesFound, orScanQR (es/en)

lib/l10n/generated/app_localizations_en.dart

@@ -512,4 +512,23 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get waitingForHost => 'Waiting for the host to start the game...';
+
+  @override
+  String get enterNameToSearch => 'Enter your name to search for nearby games';
+
+  @override
+  String get searchGames => 'Search games';
+
+  @override
+  String get searchingGames => 'Searching for nearby games...';
+
+  @override
+  String get noGamesFound => 'No games found';
+
+  @override
+  String get noGamesFoundHint =>
+      'Make sure the host has the room open and you are nearby';
+
+  @override
+  String get orScanQR => 'Not showing up? Scan the host\'s QR code';
 }