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/gui/ytsage_gui_dialogs/ytsage_dialogs_update.py

@@ -276,7 +276,7 @@ class UpdateThread(QThread):
                         return False
 
                 except subprocess.TimeoutExpired:
-                    self.update_status.emit("❌ Pip update timed out after 5 minutes")
+                    self.update_status.emit(_("update.pip_timeout"))
                     return False
                 except Exception as e:
                     self.update_status.emit(f"❌ Error during pip update: {e}")