Implement password update functionality

Allow users to update an existing password for a site. If the site is already in the password dictionary, the user should be prompted to update it with a new password.

Start by locating the password dictionary and the existing flow for adding or retrieving a site's password. Implement the update path for sites already in the dictionary, including prompting for a new password; done means an existing entry can be replaced without creating a duplicate.

Written by the indexing model from the issue text.