Custom sidebar sections

Motivation
The sidebar groups channels the way the app decides, not the way I actually work.

Proposed solution
Let people create their own named sections with an icon, drag channels between them, rename them inline, and collapse them.

The issue names no files, tests, or entry points. Start by locating the sidebar and channel-grouping implementation, then define how custom section names, icons, channel movement, inline renaming, and collapse behavior fit together; done means all of those interactions work as shown by the proposal.

Written by the indexing model from the issue text.