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/zh.json

@@ -176,7 +176,8 @@
     "please_enter_url": "请先输入网址。",
     "please_set_path": "请先设置下载路径。",
     "please_enter_url_and_path": "请输入网址并设置下载路径。",
-    "please_select_format": "请先选择格式。"
+    "please_select_format": "请先选择格式。",
+    "downloading_fallback": "⚡ 正在下载..."
   },
   "update": {
     "title": "更新 yt-dlp",
@@ -208,6 +209,7 @@
     "running_pip_install": "📦 运行 pip install --upgrade...",
     "pip_completed": "✅ pip 更新完成成功！",
     "pip_failed": "❌ pip 更新失败：{error}",
+    "pip_timeout": "❌ pip 更新超时（超过5分钟）",
     "already_up_to_date": "✅ yt-dlp已是最新版本！"
   },
   "about": {
@@ -307,7 +309,9 @@
     "analyzing_loading_thumbnail_ytdlp": "分析中 (85%)... 正在加载缩略图",
     "analyzing_processing_subtitles_ytdlp": "分析中 (90%)... 正在处理字幕",
     "select_subtitles": "选择字幕...",
-    "sponsorblock_categories": "SponsorBlock 分类..."
+    "sponsorblock_categories": "SponsorBlock 分类...",
+    "invalid_url_or_enter": "无效的URL或请输入URL。",
+    "zero_selected": "已选择 0 个"
   },
   "sponsorblock": {
     "sponsor": "赞助商",
@@ -374,7 +378,8 @@
   "playlist": {
     "unknown": "未知播放列表",
     "total_videos": "总视频数：{count}",
-    "display_format": "播放列表：{title} | {count}个视频"
+    "display_format": "播放列表：{title} | {count}个视频",
+    "select_videos_title": "选择播放列表视频"
   },
   "subtitle_selection": {
     "count_selected": "已选择{count}个"