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_en.arb

@@ -375,5 +375,19 @@
   "reconnecting": "Reconnecting...",
   "reconnectingHint": "Lost connection to the host. Your place in the game is being held.",
   "leaveGame": "Leave the game",
-  "playerRejoined": "{name} has rejoined the game"
+  "playerRejoined": "{name} has rejoined the game",
+  "leaveGameTitle": "Leave the game?",
+  "leaveGameSingleDevice": "The game in progress will be lost and cannot be recovered.",
+  "leaveGameHost": "You are the host: if you leave, the game ends for everyone.",
+  "leaveGameClient": "You will leave the game. You can rejoin while the host keeps it open.",
+  "stayInGame": "Keep playing",
+  "permissionsNeededTitle": "Permissions needed",
+  "permissionsNeededBody": "For multiplayer mode we need:",
+  "openAppSettingsQuestion": "Open the app settings?",
+  "openSettings": "Open settings",
+  "permissionLocation": "Location",
+  "permissionBluetoothAdvertise": "Bluetooth (advertise)",
+  "permissionBluetoothConnect": "Bluetooth (connect)",
+  "permissionBluetoothScan": "Bluetooth (scan)",
+  "permissionNearbyWifi": "Nearby Wi-Fi"
 }