[P9-4] Plot detail pages with prev/next navigation

[realproject7]

New route

/story/[storylineId]/[plotIndex] — individual chapter page

Layout

• Chapter header: title (or "Chapter {N}"), author address, date
• Full plot content
• Navigation buttons at bottom:
  • "Previous Chapter" (disabled on first chapter)
  • "Next Chapter" (disabled on last chapter, or show "Back to story" link)
  • "Back to Table of Contents" link
• Comments section for this specific plot (see [P9-4] Plot detail pages with prev/next navigation #187)

Navigation behavior

• Prev/Next use client-side routing for fast transitions
• URL updates to reflect current chapter: /story/2/3 = storyline 2, chapter 3
• /story/2/0 = genesis page (or redirect to main story page)

Data fetching

• Fetch single plot by storylineId + plotIndex from Supabase
• Include total plot count for prev/next button state
• SSR with revalidate for SEO

Acceptance Criteria

• Individual plot page renders at /story/[storylineId]/[plotIndex]
• Shows chapter title, author, date, full content
• Prev/Next navigation buttons work correctly
• First chapter disables Prev, last chapter disables Next
• "Back to Table of Contents" link returns to main story page
• OG metadata per chapter (title, description preview)
• Mobile responsive 375px

Blocked by: #185 (DB schema), #186 (story main page for TOC link)

Part of: Phase 9 Epic (#183)