Obviously forgot box-sizing: border-box; (or a reset stylesheet) here. Little worried about XSS, too (not for myself, but potential future users who might just paste whatever into this field), so I added a CSP that defaults to (and enforces) self. For everything. (Images are proxied anyway, and all fonts and scripts self-hosted.)
So, how’s this work? If you sign in and choose to create an account, you’ll be able to save your settings. The unique ID, here, is your Microsub URL, and settings would be stored as a JSON object (so that they can be expanded, still) in a database table. If you don’t do anything, no account will ever be made (and things continue to work the old, stateless way). The rest of the app is still powered by sessions only.