A few suggestions:
def __iter__(self):
for title, passwords in self._store.items():
yield title, passwords
This can be simplified to:
def __iter__(self):
yield from self._store.items()
Also here:
max_title_length = min(
MAX_COL_WIDTH, max(len(title) for title in self._store.keys())
)
Can be shortened by using the key argument to len:
max_title_length = min(
MAX_COL_WIDTH, len(max(self._store.keys(), key=len))
)