Desktop: sidebar section order is fixed — make it configurable (Direct messages above Channels)

**Motivation**

The desktop sidebar order is fixed: Projects (pinned above the scroll area), then Starred, custom sections, Channels, Forums, and Direct messages last. In a workspace that spawns a channel per feature, the channel list grows without bound and pushes Direct messages out of sight, so DMs are the hardest thing to reach even though they are often the most urgent. Only custom sections can be moved today; every built-in section is where it is.

**Proposed solution**

Make the order of all sidebar sections user-configurable — Direct messages, Channels, Forums, Starred, Projects and custom sections in one list — with drag-and-drop or move up/down, persisted per user.

The immediate need is being able to put Direct messages above Channels.

**Alternatives considered**

- Starring DMs so they appear in the Starred section at the top: starring only applies to channels.
- Moving DMs into a custom section: custom sections only accept channels.
- Collapsing the sections above: hides content the user still wants and does not change the order.

**Additional context**

Duplicate search: none found for section ordering. Related but different: #5253 (forum channels cannot be moved into custom sections or reordered), #2714 (threads below channels), #2667 (reorder communities in the rail).