Server-side render demo chunks for guest users' first page view

[suguanYang]

Summary

Guest users currently see an empty chunks panel on first visit. The demo document chunks are loaded client-side via SWR only after the page renders, creating a visible loading state and poor first impression.

Since demo data is identical for all guest users, we can load it server-side during page render and deliver it with the initial HTML. This eliminates the loading state and shows parsed content immediately.

Current behavior

1. User visits the notebook page as a guest
2. The sources panel shows demo documents (titles only)
3. Chunks are fetched client-side via /api/sources/{id}/chunks route, which reads from static demo files
4. User sees a loading spinner while chunks load

Proposed change

1. During SSR for guest users, call demoData.loadChunksForSource() for the first/default source
2. Pass the pre-loaded chunks as initial data prop to WorkspaceShell
3. Use SWR fallback data pattern to skip the client-side fetch when server data is available
4. The chunks data is inherently cacheable (same for all guests, read from static files)

Acceptance criteria

• Guest users see parsed chunks immediately on page load (no loading spinner)
• The existing citation click and source switching still works
• Authenticated user flow is unchanged
• Build passes and type-checks cleanly

[suguanYang]

PR open: #67