Forum channels cannot be moved into custom sidebar sections or reordered

**Describe the bug**

Stream channels can be organised into custom sidebar sections and dragged into any order. Forum channels cannot. Right-clicking a forum channel gives Copy, Mark as read, Mute, Archive and Delete, but no "Move to section", no "New section..." and no "Star channel". Right-clicking a stream channel a few rows above shows all of them.

Because the built-in groups only offer `alpha` and `recent` (`ChannelSortMode` in `desktop/src/features/sidebar/lib/channelSortPreference.ts`), and custom sections are the only place manual ordering exists, forum channels cannot be manually ordered at all.

Both groups render through the same component with different props. In `AppSidebar.tsx` the Channels group passes:

```jsx
title="Channels"
sections={channelSections}
assignments={channelAssignments}
onCreateSectionForChannel={handleCreateSectionForChannel}
```

The Forums group a few lines below passes none of those three. `ChannelContextMenu.tsx` then hides the whole submenu:

```js
const showMove = Boolean(
sections && assignments && onAssignChannel && onUnassignChannel && onCreateSectionForChannel,
);
```

Dragging is unavailable for the same structural reason: `DraggableChannelRow` is used by `CustomChannelSection.tsx`, while `SidebarSection.tsx` (which renders Channels, Forums and DMs) does not use it. So a forum channel cannot reach a custom section by dragging either.

**Steps to reproduce**

1. Right-click a stream channel in the sidebar. "Move to section" and "New section..." are present
2. Right-click a forum channel. Both are missing
3. Try dragging the forum channel onto a custom section. Nothing happens

**Expected behavior**

Forum channels should be assignable to custom sections and orderable like stream channels. Nothing in the section logic is channel-type specific, so this looks like the props were simply not wired up for the Forums group rather than a deliberate restriction.

**Version and platform**

- Buzz version: 0.5.6
- OS: Windows 11 (10.0.26200)