Mobile side nav broken when keyboard open

**Parent epic:** pskillen/meshflow-api#341

**Type:** bug | **Area:** Mobile layout

## Problem

Side navigation does not work correctly on mobile when the on-screen keyboard is open.

## Proposal

- Reproduce iOS/Android; dismiss sidebar on keyboard or fix `visualViewport` / z-index.

## Acceptance criteria

- [ ] Nav usable with keyboard open and after dismiss.
- [ ] No stuck scroll lock.