Add configurable output filename format

Introduce a new filename_format setting and UI to control yt-dlp output templates. ConfigManager now includes a default filename_format (%(title)s_%(resolution)s.%(ext)s). The DownloadSettingsDialog exposes a text input and help text for the format and saves the value to ConfigManager. DownloadThread now accepts a filename_format argument and uses it when building output templates for single videos and playlists. YTSageApp reads the config and passes the filename format into the download thread. Added corresponding English language strings.

ytsage/utils/ytsage_config_manager.py

@@ -90,6 +90,7 @@ class ConfigManager:
         "preferred_output_format": "mp4",
         "force_audio_format": False,
         "preferred_audio_format": "best",
+        "filename_format": "%(title)s_%(resolution)s.%(ext)s",
         "cached_versions": {
             "ytdlp": {"version": None, "path": None, "last_check": 0, "path_mtime": 0},
             "ffmpeg": {"version": None, "path": None, "last_check": 0, "path_mtime": 0},