Desktop: channels nested under a project show no unread or activity state

### Summary

Channels nested under a project in the sidebar render as plain navigation rows. They show no unread state, no mention badge, and no activity indicator, while the same channels listed under **Channels** (or in a user-defined section) show all three.

### What I observed

Desktop 0.5.22, self-hosted relay 0.2.1.

1. Create a project, add channels to it with **Add channel** so they appear nested under the project in the sidebar.
2. Put one of those same channels into a custom section as well (right-click → Move to section).
3. Have an agent post in that channel.

The row under **the section** shows the working indicator and the unread state. The row for the same channel **under the project** shows nothing — it looks idle whether or not work is happening.

### Where this comes from

`SidebarProjectsSection.tsx` renders its own rows and never references unread state, mention counts, or `ChannelActivityPopover`. The popover is only mounted in `SidebarSection.tsx`, which the project tree does not use.

### Why it matters

The project tree is the natural place to watch a project — it is the grouping the app itself proposes. As it stands, a user who organises work by project has to keep the same channels in a second, flat list just to see that something is happening, which defeats the grouping.

### Suggestion

Reuse the same row rendering (or at least the unread/mention/activity affordances) for project child channels, so a channel looks and behaves the same wherever it is listed.

Start by comparing SidebarProjectsSection.tsx with SidebarSection.tsx, focusing on how project child-channel rows differ from the shared channel rows. Trace the existing unread, mention, and ChannelActivityPopover handling, then verify that a nested project channel shows the same indicators as the channel in a custom section while following the reproduction steps.

Written by the indexing model from the issue text.