[FEATURE] Mods: render sites in the desktop app's empty space (sidebar bottom, chat margins)

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

I spend most of my day in the Claude desktop app with a mod board open in the right pane: live sessions, open asks, lane progress. The board runs out of room fast. Meanwhile two large parts of the window stay empty. The bottom half of the left sidebar, below the session list, is blank. So are the margins on both sides of the chat column once the window is wide.

A mod can draw only in AbovePrompt, in a Pane, and in the engine's own message rows (the RenderComponent sites on 2.1.296). None of that empty space is reachable.

Proposed Solution

Two new render sites on the desktop surface. A SidebarFooter slot under the session list, sized to the free space. And ChatMargin slots to the left and right of the conversation column, drawn only when the window is wide enough to have margins. Both could follow the rules a Pane already has: size from bodyColumns, and simply absent on surfaces that lack them.

Alternative Solutions

Several panes at once (#101525) would help, but it splits the right pane further instead of using space that is already free. Today I fold everything into one pane and collapse sections by hand.

Priority

Medium - Would be very helpful

Feature Category

Other (plugins / mods, desktop app)

Use Case Example

The open-asks list lives in the sidebar footer, so it stays visible while the right pane shows a file, the browser or a diff. A narrow lane-progress strip sits in a chat margin.

Additional Context

Labels I would pick: enhancement, area:plugins, area:desktop. Related: #101525.

— printed by Claude Code, signed by me.