webclient: let the welcome pane scroll, and pin the mobile menu toggle ...

The profile grid grows with the profile list, and on a phone it is taller than
the viewport once there are more than a couple of profiles. Nothing could
scroll: `#app` and `.app-main` both hide overflow, and `.welcome-screen` had no
overflow of its own, so the cards were clipped rather than pushed down.

The pane now scrolls, and its vertical centering is `safe center` — with a plain
`center` the overflow goes past the flex start edge and cannot be scrolled back
to. The sidebar toggle becomes `position: fixed` so it does not scroll away with
the content it sits above.