Fix build error by moving zine.md out of content/ directory

Astro treats src/content/ as a content collection directory, which
caused a build error. Moved zine.md to src/data/ and updated
references in index.astro and README.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -43,7 +43,7 @@ Node.js >= 18.20.8 (or upgrade to Node.js 20+ for best compatibility)
 
 ```
 ├── src/
-│   ├── content/
+│   ├── data/
 │   │   └── zine.md          # Your zine content in Markdown
 │   └── pages/
 │       └── index.astro      # Main page that renders the zine
@@ -53,7 +53,7 @@ Node.js >= 18.20.8 (or upgrade to Node.js 20+ for best compatibility)
 
 ## Editing Your Zine
 
-1. Open `src/content/zine.md`
+1. Open `src/data/zine.md`
 2. Write your content using Markdown
 3. Use `## Heading` for section breaks (each becomes a full-height section)
 4. The dev server will hot-reload your changes