fix video embeds: use clickable thumbnails instead of iframes

Marp renders slides inside SVG foreignObject which blocks YouTube
iframe players (Error 153). Switch to large clickable thumbnail
images (maxresdefault) with rounded corners and drop shadow that
open the video in a new tab on click.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jeremymanning

slides/data-story-workshop.md

@@ -95,15 +95,15 @@ Your audience came for a **story**, not a statistics lecture. Data should be the
 # Example: Hans Rosling — 200 countries, 200 years, 4 minutes
 
 <div style="text-align: center;">
-<iframe width="800" height="450" src="https://www.youtube.com/embed/jbkSRLYSojo" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
+<a href="https://www.youtube.com/watch?v=jbkSRLYSojo" target="_blank"><img src="https://img.youtube.com/vi/jbkSRLYSojo/maxresdefault.jpg" style="width: 85%; max-width: 900px; border-radius: 12px; box-shadow: 0 4px 12px rgba(0,0,0,0.2);"></a>
 </div>
 
 ---
 
 # Example: Powers of Ten (1977)
 
 <div style="text-align: center;">
-<iframe width="800" height="450" src="https://www.youtube.com/embed/0fKBhvDjuy0" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
+<a href="https://www.youtube.com/watch?v=0fKBhvDjuy0" target="_blank"><img src="https://img.youtube.com/vi/0fKBhvDjuy0/maxresdefault.jpg" style="width: 85%; max-width: 900px; border-radius: 12px; box-shadow: 0 4px 12px rgba(0,0,0,0.2);"></a>
 </div>
 
 ---