Client Login Page and Authentication

The client should have a login page that accepts the user's username, password, and secret key. The client should then use the authorization flow implemented in #3 to obtain access to the user's account. On authorization completion, basic user account information (or any errors) should be appended below the form.

Non-goals:
1. Credential caching
2. Logged in screen(s)
3. Polished styling
4. Light/dark modes (one is sufficient)

Start by reading the authorization flow implemented in issue #3 and locating the client entry point for the login form. Confirm that the form accepts a username, password, and secret key, then verify that successful authorization appends basic account information and failures append errors below it; credential caching and polished styling are out of scope.

Written by the indexing model from the issue text.