fix: remove singleton flag from homepage collection

Singleton collections don't support live editing in EmDash.
Converting to regular collection enables live editing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

seed/seed.json

@@ -20,7 +20,6 @@
 				"revisions",
 				"search"
 			],
-			"singleton": true,
 			"fields": [
 				{
 					"slug": "hero_headline",