Allow reordering custom session groups in the sidebar (drag-to-reorder)

What

Add the ability to manually reorder custom session groups in the left sidebar (Group by → Custom groups), e.g. via drag-and-drop of the group header, or by exposing a group-level sort.

Current behavior

With Group by: Custom groups, the group headers display in what appears to be group creation order, and nothing in the UI re-sorts them:

• The Sort by menu (Alphabetically / Created time / Recency) only sorts sessions within each group — it has no effect on the order of the group headers.
• Group headers cannot be dragged to reorder (drag only moves sessions into a group).
• Renaming groups with numeric prefixes ([0], [5], [9] …) does not reorder them, since no sort applies to groups.

This looks related to #56060 (Sort by has no effect when Group by: Project), but is specifically about custom groups.

Expected behavior

Users can control the order of their custom groups — ideally drag-to-reorder the group headers, with the order persisted. A group-level "Sort by" (or having the existing Sort by apply to groups too) would also solve it.

Why

Custom groups are used to organize many parallel sessions by priority/topic. Without control over group order, the most-used groups can end up buried below rarely-touched ones, and there's no workaround (prefix trick only affects sessions inside a group).

Environment

• Claude Code 2.1.170
• Desktop app, left nav panel, Group by: Custom groups