Refactor UI startup and stylesheet handling

Moved the main application stylesheet to a new src/gui/ytsage_stylesheet.py module and applied it via StyleSheet.MAIN. Deferred blocking startup checks (FFmpeg, yt-dlp, Deno, updates) until after the UI is shown to improve responsiveness. Improved subtitle file deletion logic and added destination filename capture in downloader. Added custom style for auto-update checkbox in updater dialog.

main.py

@@ -3,8 +3,6 @@ import sys
 from PySide6.QtWidgets import QApplication, QMessageBox
 
 from src.utils.ytsage_logger import logger
-from src.core.ytsage_yt_dlp import check_ytdlp_binary, setup_ytdlp  # Import the new yt-dlp setup functions
-from src.core.ytsage_deno import check_deno_binary, setup_deno  # Import the new Deno setup functions
 from src.gui.ytsage_gui_main import YTSageApp  # Import the main application class from ytsage_gui_main
 
 
@@ -22,21 +20,6 @@ def main():
         logger.info("Starting YTSage application")
         app = QApplication(sys.argv)
 
-        # Get the expected binary path and check if it exists
-        if not check_ytdlp_binary():
-            # No app-specific binary found, show setup dialog regardless of Python package
-            logger.warning("No yt-dlp binary found, starting setup process")
-            yt_dlp_path = setup_ytdlp()
-            if yt_dlp_path == "yt-dlp":  # If user canceled or something went wrong
-                logger.warning("yt-dlp not configured properly")
-
-        # Check for Deno binary
-        if not check_deno_binary():
-            logger.warning("No Deno binary found, starting setup process")
-            deno_path = setup_deno()
-            if deno_path == "deno":  # If user canceled or something went wrong
-                logger.warning("Deno not configured properly")
-
         window = YTSageApp()  # Instantiate the main application class
         window.show()
         logger.info("Application window shown, entering main loop")