feat(layout): add channels unread badge to sidebar

[prellr]

Summary

• Adds an unread message indicator to the Channels nav item in the app sidebar, matching the visual treatment used by Ship (amber dot + count)
• Queries the channels list when channels_enabled is set; sums unread_count across all channels
• Shows ● N badge (amber dot + number, capped at 99+) next to Channels when there are unread messages; hidden otherwise
• No badge rendered when channels feature is disabled or all channels are read

How it works

The channelsListOptions query (already used by the channels page) is fetched in the sidebar with the same channels_enabled guard. The sidebar already fetches Ship data the same way. The Channel.unread_count field is populated by the backend and optimistically cleared by useMarkChannelRead when the user views a channel.

Test plan

• Open a workspace with channels enabled and unread messages — verify amber dot + count appears next to Channels in sidebar
• Mark channels as read — verify badge disappears
• Workspace with channels disabled — verify no query fires and no badge appears
• Count exceeds 99 — verify "99+" is displayed

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