Add a recent posts home page

[benvinegar]

Summary

Add a default Home view that shows the latest posts across the local sideshow workspace, so users can quickly see recent agent output without first choosing a session from the sidebar.

Motivation

The current viewer is session-first. That works once you know where to look, but a recent-posts landing page would make sideshow feel more alive on open and help users rediscover recent work across sessions.

Sketch

• Add a routed Home view for the workspace root.
• Fetch recent posts from the existing recent-posts API.
• Render each post as a compact card with:
  • session title
  • post title / agent / relative timestamp
  • part-kind summary
  • a clipped preview of the real rendered surface content
• Clicking a card should deep-link to the full post in its session.
• Keep the implementation generic for self-hosted/local usage.

Acceptance criteria

• Opening the root viewer path shows a Home/recent-posts view when posts exist.
• Recent posts link to their full session/post view.
• The route remains stable and does not get auto-replaced by session auto-selection.
• Tests cover empty, populated, and direct-link behavior.