Add localization keys for fallback and status messages in UI

Replaced hardcoded status and fallback messages with localized strings across downloader, update, and main UI modules. Updated language files for all supported languages to include new keys for fallback download status, pip timeout, invalid URL, zero selected, and playlist selection dialog title. This improves internationalization and consistency of user-facing messages.

languages/ja.json

@@ -176,7 +176,8 @@
     "please_enter_url": "最初にURLを入力してください。",
     "please_set_path": "最初にダウンロードパスを設定してください。",
     "please_enter_url_and_path": "URLを入力し、ダウンロードパスを設定してください。",
-    "please_select_format": "最初にフォーマットを選択してください。"
+    "please_select_format": "最初にフォーマットを選択してください。",
+    "downloading_fallback": "⚡ ダウンロード中..."
   },
   "update": {
     "title": "yt-dlpを更新",
@@ -208,7 +209,8 @@
     "running_pip_install": "📦 pip install --upgradeを実行中...",
     "pip_completed": "✅ pipの更新が正常に完了しました！",
     "pip_failed": "❌ pipの更新に失敗しました: {error}",
-    "already_up_to_date": "✅ yt-dlpはすでに最新です！"
+    "already_up_to_date": "✅ yt-dlpはすでに最新です！",
+    "pip_timeout": "❌ pip更新が5分後にタイムアウトしました"
   },
   "about": {
     "title": "YTSageについて",
@@ -307,7 +309,9 @@
     "analyzing_loading_thumbnail_ytdlp": "解析中 (85%)... サムネイルを読み込み中",
     "analyzing_processing_subtitles_ytdlp": "解析中 (90%)... 字幕を処理中",
     "select_subtitles": "字幕を選択...",
-    "sponsorblock_categories": "SponsorBlockカテゴリ..."
+    "sponsorblock_categories": "SponsorBlockカテゴリ...",
+    "invalid_url_or_enter": "無効なURLまたはURLを入力してください。",
+    "zero_selected": "0個選択"
   },
   "sponsorblock": {
     "sponsor": "スポンサー",
@@ -374,7 +378,8 @@
   "playlist": {
     "unknown": "不明な再生リスト",
     "total_videos": "総動画数: {count}",
-    "display_format": "再生リスト: {title} | {count}個の動画"
+    "display_format": "再生リスト: {title} | {count}個の動画",
+    "select_videos_title": "プレイリスト動画を選択"
   },
   "subtitle_selection": {
     "count_selected": "{count}個選択"