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/app_en.arb

@@ -249,5 +249,11 @@
   "scanQR": "Scan QR",
   "scanHostQR": "Point at the host's QR code",
   "connectedWaiting": "Connected!",
-  "waitingForHost": "Waiting for the host to start the game..."
+  "waitingForHost": "Waiting for the host to start the game...",
+  "enterNameToSearch": "Enter your name to search for nearby games",
+  "searchGames": "Search games",
+  "searchingGames": "Searching for nearby games...",
+  "noGamesFound": "No games found",
+  "noGamesFoundHint": "Make sure the host has the room open and you are nearby",
+  "orScanQR": "Not showing up? Scan the host's QR code"
 }