fix: correct YAML frontmatter, convert images to proper WebP format

- Moved image: field from outside to inside YAML frontmatter (50 files)
- Converted all 56 non-WebP images to proper WebP format via ImageMagick
- Fixed Python shebangs for portability
- Updated source fields in 2 Task 3 recipes

bin/task3-frontmatter.py

@@ -1,4 +1,4 @@
-#!/tmp/recipe-venv/bin/python3
+#!/usr/bin/env python3
 """Add frontmatter and images to 9 recipes without frontmatter."""
 
 import re, sys, time