[FEATURE] Desktop app: let the Ungrouped sidebar section be moved and collapsed

## Problem Statement

In the desktop app's left-hand session list the group sections are in a fixed order, and **Ungrouped** can neither be moved nor collapsed. Anyone with a lot of ungrouped sessions therefore has every named group pushed far below the fold, and reaching them means scrolling past a list that is never going to be read.

## Proposed Solution

Either of these fixes it:

- let the **Ungrouped** section be dragged like any other group — in particular to sit directly under **Pinned** — and let it be collapsed; or
- let **Pinned** be moved down, so the sections above it can be reordered around it.

Making Ungrouped collapsible on its own would already remove most of the problem.

## Alternative Solutions

Putting every session into a named group, which is work that exists only to keep the sidebar usable.

## Additional Context

Purely a sidebar layout issue in the desktop app; it does not affect the CLI.

Start by locating the desktop app’s left-hand session-list/sidebar implementation and the existing group drag and collapse behavior. Because the proposal offers alternative fixes, confirm which behavior maintainers want before implementation. Done means the selected Ungrouped or Pinned behavior works in the sidebar without affecting the CLI.

索引モデルが issue の本文から書いたものです。