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_ja.arb

@@ -246,28 +246,28 @@
   "release": "解除",
   "select": "選択",
   "delete": "削除",
-  "selectAtLeastThreeUsersToStart": "Select at least 3 users to start.",
-  "hostPhoneMustSelectUser": "The host phone must select at least one user.",
-  "roomNoLongerInLobby": "The room is no longer in the lobby.",
-  "completeUserSelectionToStart": "Complete user selection to start.",
-  "preparingSecureRoom": "Preparing the secure room",
-  "searchingNearbyBluetoothGames": "Searching nearby games over Bluetooth",
-  "tapToJoin": "Tap to join",
-  "bluetoothLocationPermissionsRequired": "Bluetooth and location permissions are required to search for games.",
-  "bluetoothLocationPermissionsShort": "Bluetooth and location permissions are required",
-  "couldNotStartSearch": "Could not start the search. Check Bluetooth and location.",
-  "couldNotConnectToHost": "Could not connect to {host}",
+  "selectAtLeastThreeUsersToStart": "開始するには3人以上のユーザーを選んでください。",
+  "hostPhoneMustSelectUser": "ホストの端末は少なくとも1人のユーザーを選ぶ必要があります。",
+  "roomNoLongerInLobby": "ルームはすでにロビーにありません。",
+  "completeUserSelectionToStart": "開始するにはユーザーの選択を完了してください。",
+  "preparingSecureRoom": "安全なルームを準備しています",
+  "searchingNearbyBluetoothGames": "Bluetoothで近くのゲームを検索しています",
+  "tapToJoin": "タップして参加",
+  "bluetoothLocationPermissionsRequired": "ゲームを検索するにはBluetoothと位置情報の許可が必要です。",
+  "bluetoothLocationPermissionsShort": "Bluetoothと位置情報の許可が必要です",
+  "couldNotStartSearch": "検索を開始できませんでした。Bluetoothと位置情報を確認してください。",
+  "couldNotConnectToHost": "{host} に接続できませんでした",
   "room": "ルーム",
-  "singleDeviceSubtitle": "Game on this device",
-  "singleDeviceDescription": "Ideal for playing together by passing the phone around. Fast, direct setup.",
-  "multiDeviceSubtitle": "Each player on their phone",
-  "multiDeviceDescription": "Create a premium room, share the QR code and manage users from the lobby.",
-  "singleDeviceGameLabel": "Game on this device",
-  "multiDeviceGameLabel": "Multi-device game",
-  "mainDeviceUser": "Main device user",
-  "couldNotCreateRoom": "Could not create the room. Check Bluetooth.",
-  "cannotStartWithReason": "Cannot start: {reason}",
-  "invalidRoom": "invalid room",
+  "singleDeviceSubtitle": "この端末でプレイ",
+  "singleDeviceDescription": "端末を回しながらみんなで遊ぶのに最適。設定はすばやく簡単。",
+  "multiDeviceSubtitle": "プレイヤーごとに自分の端末で",
+  "multiDeviceDescription": "プレミアムなルームを作り、QRコードを共有し、ロビーからユーザーを管理します。",
+  "singleDeviceGameLabel": "この端末でのゲーム",
+  "multiDeviceGameLabel": "マルチデバイスのゲーム",
+  "mainDeviceUser": "この端末のメインユーザー",
+  "couldNotCreateRoom": "ルームを作成できませんでした。Bluetoothを確認してください。",
+  "cannotStartWithReason": "開始できません: {reason}",
+  "invalidRoom": "無効なルーム",
   "defaultPlayerName": "プレイヤー",
   "play": "プレイ",
   "history": "履歴",
@@ -368,5 +368,19 @@
   "reconnecting": "再接続しています...",
   "reconnectingHint": "ホストとの接続が切れました。ゲーム内のあなたの席は確保されています。",
   "leaveGame": "ゲームから退出",
-  "playerRejoined": "{name} がゲームに復帰しました"
+  "playerRejoined": "{name} がゲームに復帰しました",
+  "leaveGameTitle": "ゲームから退出しますか？",
+  "leaveGameSingleDevice": "進行中のゲームは失われ、元に戻せません。",
+  "leaveGameHost": "あなたはホストです。退出すると全員のゲームが終了します。",
+  "leaveGameClient": "ゲームから退出します。ホストが開いている間は戻れます。",
+  "stayInGame": "ゲームを続ける",
+  "permissionsNeededTitle": "必要な権限",
+  "permissionsNeededBody": "マルチプレイには次の権限が必要です：",
+  "openAppSettingsQuestion": "アプリの設定を開きますか？",
+  "openSettings": "設定を開く",
+  "permissionLocation": "位置情報",
+  "permissionBluetoothAdvertise": "Bluetooth（アドバタイズ）",
+  "permissionBluetoothConnect": "Bluetooth（接続）",
+  "permissionBluetoothScan": "Bluetooth（スキャン）",
+  "permissionNearbyWifi": "近くのWi-Fi"
 }