Upgrade flutter version by dropping use of dart:html in confapp

### Motivation

We cannot upgrade past 3.27 without running into deprecation of dart:html.
This affects not only some of the widgets, but also the Worker class used to run yosys.

### Desired solution

package:web is the proposed alternative, and this will require some rewriting of the Flutter code in confapp

### Alternatives considered

_No response_

### Additional details

_No response_

Start in the confapp Flutter code by locating each dart:html import and the Worker class used to run yosys. Review the package:web migration requirements, then verify the app builds on Flutter 3.27+ without dart:html and that the affected widgets and yosys worker still work.

Written by the indexing model from the issue text.