UI: "Group By" in workflows list

# Summary

In the interest of improving readability and page performance with a large number of workflows, I think it would be useful to have a "group by" feature in the workflows list.

# Motivation

When a user has hundreds of workflows it would be useful to have an additional way to sort them. Performance could also be improved (without having to use pagination) by collapsing / hiding groups so that fewer items are rendered at a time.

# Proposal


I think it would look something like this (rough vector mockup). The group selector could naturally be added to the sidebar instead, but I think it might make sense to have it in the top bar nearby the batch selection actions to keep features designed for working with many workflows together. Something I didn't include in this mockup is an arrow near the group label to show/hide each group.

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**Message from the maintainers**:

If you wish to see this enhancement implemented please add a 👍 reaction to this issue! We often sort issues this way to know what to prioritize.

No source files, tests, or entry points are named. Start by locating the workflows list UI and reviewing how list actions and rendering are organized; done means an agreed grouping control, collapsible groups, and verified usability and performance for hundreds of workflows.

Written by the indexing model from the issue text.