Add new translation keys for file exists and auto-update dialogs

Introduces new translation keys for file exists and auto-update dialogs in all supported languages. Updates the settings and main GUI to use these i18n strings instead of hardcoded English text, improving localization and user experience.

languages/ja.json

@@ -43,7 +43,6 @@
     "will_merge_audio": "音声が結合されます",
     "has_audio": "✓ 音声あり",
     "audio_only": "音声のみ",
-
     "best_4k": "最高 (4K)",
     "best_2k": "最高 (2K)",
     "high_1080p": "高 (1080p)",
@@ -59,7 +58,7 @@
   "buttons": {
     "download": "ダウンロード",
     "pause": "一時停止",
-    "resume": "再開", 
+    "resume": "再開",
     "cancel": "キャンセル",
     "browse": "参照",
     "clear": "クリア",
@@ -96,7 +95,7 @@
   "tabs": {
     "cookies": "Cookieでログイン",
     "custom_command": "カスタムコマンド",
-    "proxy": "プロキシ", 
+    "proxy": "プロキシ",
     "language": "言語"
   },
   "cookies": {
@@ -376,5 +375,21 @@
   },
   "subtitle_selection": {
     "count_selected": "{count}個選択"
+  },
+  "file_exists_dialog": {
+    "title": "ファイルが既に存在します",
+    "message": "ファイルが既に存在します:\n{filename}",
+    "info": "この動画は既にダウンロードされています。"
+  },
+  "auto_update": {
+    "last_check_never": "最終更新確認: なし",
+    "last_check": "最終更新確認: {time}",
+    "next_check_disabled": "次回確認: 無効",
+    "next_check_startup": "次回確認: 起動時",
+    "next_check_overdue": "次回確認: 今すぐ (遅延)",
+    "next_check": "次回確認: {time}",
+    "next_check_error": "次回確認: 計算エラー",
+    "checking": "🔄 確認中...",
+    "check_now": "🔍 今すぐ更新を確認"
   }
 }