Add Feed and Post Detail UI

[cameronhotchkies]

Build the React UI for browsing the post feed and viewing post detail pages.

✅ Acceptance Criteria

• Homepage displays posts in newest-first order
• Each post in feed shows title, author, and timestamp
• Clicking a post navigates to /posts/{id} detail page
• Detail page renders post body as formatted HTML
• Logged-out users can browse feed and view posts (read-only)

Internal Coding

MR1

• Feed component with post list
• Post card component
• Post detail page
• React Router setup