Item detail RSC page with recursive comments + toggle

[justin808]

Goal

Implement the item detail page with recursive comments and client-side collapse/expand behavior.

Scope

• Item detail RSC page
• Recursive comment rendering
• Client comment-toggle component
• Error handling for missing/invalid item IDs
• Document handling of HN HTML comment content

Acceptance Criteria

• Item page renders story metadata + nested comments
• Comment collapse/expand works per-thread
• Invalid item IDs return a clear not-found/error state