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

Kale and Chestnut Stew.cook

@@ -1,4 +1,3 @@
-image: https://roux.roo.lol/recipe-images/kale-and-chestnut-stew.webp
 ---
 title: Kale & Chestnut Stew with Butternut Squash, Wild Rice & Parsley–Walnut Pesto
 servings: 2
@@ -9,6 +8,7 @@ source: Garnish & Gather — Chef Emily Golub
 description: Cozy kale-and-chestnut stew finished with cream and topped with a bright parsley–walnut pesto. Allergens: dairy, walnuts.
 prep_time: 20 minutes
 cook_time: 35 minutes
+image: https://roux.roo.lol/recipe-images/kale-and-chestnut-stew.webp
 ---
 
 Remove any large stems from @kale{3 c} and thinly slice into ribbons.