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/en.json

@@ -176,7 +176,8 @@
     "please_enter_url": "Please enter a URL first.",
     "please_set_path": "Please set a download path first.",
     "please_enter_url_and_path": "Please enter a URL and set download path.",
-    "please_select_format": "Please select a format first."
+    "please_select_format": "Please select a format first.",
+    "downloading_fallback": "⚡ Downloading..."
   },
   "update": {
     "title": "Update yt-dlp",
@@ -208,7 +209,8 @@
     "updating_from_to": "⬆️ Updating from {current} to {latest}...",
     "running_pip_install": "📦 Running pip install --upgrade...",
     "pip_completed": "✅ Pip update completed successfully!",
-    "pip_failed": "❌ Pip update failed: {error}"
+    "pip_failed": "❌ Pip update failed: {error}",
+    "pip_timeout": "❌ Pip update timed out after 5 minutes"
   },
   "about": {
     "title": "About YTSage",
@@ -307,7 +309,9 @@
     "analyzing_loading_thumbnail_ytdlp": "Analyzing (85%)... Loading thumbnail",
     "analyzing_processing_subtitles_ytdlp": "Analyzing (90%)... Processing subtitles",
     "select_subtitles": "Select Subtitles...",
-    "sponsorblock_categories": "SponsorBlock Categories..."
+    "sponsorblock_categories": "SponsorBlock Categories...",
+    "invalid_url_or_enter": "Invalid URL or please enter a URL.",
+    "zero_selected": "0 selected"
   },
   "sponsorblock": {
     "sponsor": "Sponsor",
@@ -374,7 +378,8 @@
   "playlist": {
     "unknown": "Unknown Playlist",
     "total_videos": "Total Videos: {count}",
-    "display_format": "Playlist: {title} | {count} videos"
+    "display_format": "Playlist: {title} | {count} videos",
+    "select_videos_title": "Select Playlist Videos"
   },
   "subtitle_selection": {
     "count_selected": "{count} selected"