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.

src/core/ytsage_downloader.py

@@ -568,7 +568,7 @@ class DownloadThread(QThread):
                     self.status_signal.emit(_("download.downloading"))
             except Exception as e:
                 logger.exception(f"Error extracting filename from line '{line}': {e}")
-                self.status_signal.emit("⚡ Downloading...")  # Fallback status
+                self.status_signal.emit(_("download.downloading_fallback"))  # Fallback status
             return  # Don't process this line further for speed/ETA
 
         # Check for specific download types in the output
@@ -693,7 +693,7 @@ class DownloadThread(QThread):
                 self.file_exists_signal.emit(filename)
             else:
                 logger.info(f"Could not extract filename from 'already downloaded' line: {line}")
-                self.status_signal.emit("⚠️ File already exists")  # Fallback status
+                self.status_signal.emit(_("download.file_exists"))  # Fallback status
         elif "Finished downloading" in line:
             self.progress_signal.emit(100)