Persist and restore main window state

Restore window geometry/state on startup and save them on exit. Adds YTSageApp._load_window_state() and calls it during UI init; saves Base64-encoded geometry/state via ConfigManager when closing. Adds default config keys "window_geometry" and "window_state". Includes error handling and debug logging to avoid failures if stored values are invalid.

ytsage/utils/ytsage_config_manager.py

@@ -96,6 +96,8 @@ class ConfigManager:
         "preferred_audio_format": "best",
         "audio_normalization": False,
         "filename_format": "%(title)s_%(resolution)s_[%(id)s].%(ext)s",
+        "window_geometry": None,
+        "window_state": None,
         "cached_versions": {
             "ytdlp": {"version": None, "path": None, "last_check": 0, "path_mtime": 0},
             "ffmpeg": {"version": None, "path": None, "last_check": 0, "path_mtime": 0},