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/generated/app_localizations_en.dart

@@ -840,4 +840,49 @@ class AppLocalizationsEn extends AppLocalizations {
   String playerRejoined(String name) {
     return '$name has rejoined the game';
   }
+
+  @override
+  String get leaveGameTitle => 'Leave the game?';
+
+  @override
+  String get leaveGameSingleDevice =>
+      'The game in progress will be lost and cannot be recovered.';
+
+  @override
+  String get leaveGameHost =>
+      'You are the host: if you leave, the game ends for everyone.';
+
+  @override
+  String get leaveGameClient =>
+      'You will leave the game. You can rejoin while the host keeps it open.';
+
+  @override
+  String get stayInGame => 'Keep playing';
+
+  @override
+  String get permissionsNeededTitle => 'Permissions needed';
+
+  @override
+  String get permissionsNeededBody => 'For multiplayer mode we need:';
+
+  @override
+  String get openAppSettingsQuestion => 'Open the app settings?';
+
+  @override
+  String get openSettings => 'Open settings';
+
+  @override
+  String get permissionLocation => 'Location';
+
+  @override
+  String get permissionBluetoothAdvertise => 'Bluetooth (advertise)';
+
+  @override
+  String get permissionBluetoothConnect => 'Bluetooth (connect)';
+
+  @override
+  String get permissionBluetoothScan => 'Bluetooth (scan)';
+
+  @override
+  String get permissionNearbyWifi => 'Nearby Wi-Fi';
 }