Support Markdown rendering for story content + Write/Preview toggle

[realproject7]

Summary

Add Markdown support for story content — both rendering on the story page and a Write/Preview toggle on the writing interface.

1. Markdown rendering on story pages

Display story content as rendered Markdown instead of plain text.

• Use react-markdown (lightweight, widely used)
• Support basic Markdown only — suited for fiction writing:
  • Bold, italic, strikethrough
  • # Headings (h1-h3) for chapter sections
  • > Blockquotes for dialogue, inner thoughts, letters
  • --- for scene breaks
  • Paragraph spacing (double newline)
  • Bullet/numbered lists
  • inline code for in-story emphasis (e.g. terminal text in sci-fi)
• Sanitize output — strip raw HTML, scripts, iframes to prevent XSS. Use rehype-sanitize or similar
• No images, tables, or code blocks — not relevant for fiction, keep it simple
• Style rendered Markdown to match PlotLink's moleskine/prose aesthetic (Lora font for body, proper spacing)
• Existing plain text stories render fine — plain text is valid Markdown

2. Write/Preview toggle on writing interface

Add a GitHub-style Write/Preview tab toggle on both the Create Storyline and Chain Plot forms.

• Write tab: Current textarea for writing content
• Preview tab: Renders the textarea content as Markdown in real-time
• Toggle between tabs without losing content
• Preview should match the exact styling of the story page rendering
• Reference: GitHub's issue/PR editor Write/Preview toggle (see screenshot in ticket)

Files

• Story page plot rendering: src/app/story/[storylineId]/page.tsx (or wherever plot content is rendered)
• Create storyline form: src/app/create/page.tsx
• Chain plot form: wherever the chain plot UI lives
• New dependency: react-markdown + rehype-sanitize

Acceptance Criteria

• Story page renders Markdown content (bold, italic, headings, blockquotes, scene breaks, lists)
• Raw HTML/scripts are sanitized — no XSS
• Write/Preview toggle on create storyline form
• Write/Preview toggle on chain plot form
• Preview matches story page rendering style exactly
• Existing plain text stories still render correctly
• Markdown styling matches PlotLink prose aesthetic (Lora font, proper spacing)