Make config persistence robust and history DB concurrency-safe

- Config saves are atomic (temp file + fsync + os.replace); a crash or
  power loss mid-write no longer truncates the file, which previously
  caused a silent reset to defaults on next launch.
- Stored config is merged over a deep copy of the defaults: keys added
  in newer versions resolve properly instead of returning None, and the
  nested cached_versions dict is no longer shared with (and mutated on)
  the class-level default dict.
- History SQLite connection enables WAL and a 5s busy timeout so the
  download thread can record entries while the history dialog reads
  without "database is locked" errors.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ytsage/utils/ytsage_history_manager.py

@@ -72,6 +72,10 @@ class HistoryManager:
                 if cls._connection is None:
                     cls._connection = sqlite3.connect(cls._db_file, check_same_thread=False)
                     cls._connection.row_factory = sqlite3.Row
+                    # WAL lets the download thread write while the history
+                    # dialog reads; busy_timeout avoids "database is locked"
+                    cls._connection.execute("PRAGMA journal_mode=WAL")
+                    cls._connection.execute("PRAGMA busy_timeout=5000")
                 
                 cursor = cls._connection.cursor()