Wire testimonials CMS collection and Payload Live Preview

Testimonials on /todo-cards, /newsletter, /challenge now come from the new
Payload testimonials collection via a shared TestimonialsGrid component,
instead of 3 separately hardcoded arrays.

Adds Next.js Draft Mode (/api/preview) plus Live-Preview-aware client
wrappers (LiveRichText, LiveTestimonialsGrid, LivePostContent) for posts,
legal pages, and testimonials — mounted only while Draft Mode is enabled,
so ordinary visitors keep getting the plain static components.

package.json

@@ -9,6 +9,7 @@
     "lint": "eslint"
   },
   "dependencies": {
+    "@payloadcms/live-preview-react": "^3.85.2",
     "motion": "^12.42.2",
     "next": "16.2.9",
     "react": "19.2.4",