Allow reordering / alphabetical sorting of sidebar groups (desktop)

What
In the desktop app, sessions can be organized into groups (via "Move to group"). The groups appear in the sidebar in creation order, and there is currently no way to change that order.

Request
Add the ability to reorder groups — ideally both:

• manual drag-and-drop reordering of groups, and/or
• a "sort groups alphabetically" option.

Note: This is specifically about the order of the groups themselves, not the sessions inside a group.

Why
With more than a handful of groups, creation order becomes hard to scan. Alphabetical ordering makes a group instantly findable.

Related but distinct: #25974 covers pinning/reordering of sessions; this issue is about the group level.