Top bar scrolls out of view when mobile keyboard opens

## Problem

On mobile, the top bar (header/navigation) is normally pinned and visible. However, **when the on-screen keyboard opens** (e.g. when focusing an input field to type a message), the top bar gets pushed out of view.

This means the user loses access to navigation and controls while typing, which is precisely when they're actively interacting with the app.

## Expected behavior

The top bar should remain pinned to the top of the viewport even when the on-screen keyboard opens, so navigation and controls stay accessible while typing.

## Context

- This is specifically a mobile issue triggered by the keyboard appearing.
- Likely related to how the viewport/layout responds to the keyboard (e.g. visual viewport API, `position: sticky` vs `fixed`, or lack of handling for `resize`/`visualViewport` events).
- Also related to #64 (general mobile responsiveness of the workspace view).

Locate the top bar/header and the mobile workspace layout, then reproduce the issue by focusing an input and opening the on-screen keyboard on a mobile viewport. Check how the layout responds to viewport changes; done means the top bar remains pinned and its navigation and controls stay accessible while typing.

Written by the indexing model from the issue text.