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.

lib/l10n/app_es.arb

@@ -417,5 +417,19 @@
         "type": "String"
       }
     }
-  }
+  },
+  "leaveGameTitle": "¿Salir de la partida?",
+  "leaveGameSingleDevice": "Se perderá la partida en curso y no podrás recuperarla.",
+  "leaveGameHost": "Eres el anfitrión: si sales, la partida termina para todos los jugadores.",
+  "leaveGameClient": "Saldrás de la partida. Podrás volver mientras el anfitrión la mantenga abierta.",
+  "stayInGame": "Seguir jugando",
+  "permissionsNeededTitle": "Permisos necesarios",
+  "permissionsNeededBody": "Para el modo multijugador necesitamos:",
+  "openAppSettingsQuestion": "¿Abrir la configuración de la app?",
+  "openSettings": "Abrir configuración",
+  "permissionLocation": "Ubicación",
+  "permissionBluetoothAdvertise": "Bluetooth (anunciar)",
+  "permissionBluetoothConnect": "Bluetooth (conectar)",
+  "permissionBluetoothScan": "Bluetooth (escanear)",
+  "permissionNearbyWifi": "Wi-Fi cercano"
 }