Allow users to transfer credits themselves from the dashboard from one account to another

Currently, users are not able to transfer prepaid credits from one account to another; this feature will help them do it from their end.

Start by locating the dashboard flow for account and prepaid-credit management; the issue does not name a file, entry point, or test. Trace how accounts and credits are represented, then define the transfer rules and add a user-facing flow whose completed state confirms that credits moved between the two accounts.

Written by the indexing model from the issue text.