Add a way to save completed problems

[veeral-patel]

Hi - if you could log in or download and later upload the list of problems you completed, instead of just storing in localstorage, that would be a great improvement and also allow users to use the project across devices.

[seanprashad]

Duplicate of #155

[yashjakhotiya]

Not necessarily a duplicate. leetcode.com has a concept of "sessions", which people can use to restart their leetcode journey. Most people use it share LC premium. It could be helpful in this scenario to have a different login identity for leetcode-patterns than leetcode.com.