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-lock.json

@@ -8,6 +8,7 @@
       "name": "einfach-produktiv",
       "version": "0.1.0",
       "dependencies": {
+        "@payloadcms/live-preview-react": "^3.85.2",
         "motion": "^12.42.2",
         "next": "16.2.9",
         "react": "19.2.4",
@@ -1307,6 +1308,25 @@
         "node": ">=12.4.0"
       }
     },
+    "node_modules/@payloadcms/live-preview": {
+      "version": "3.85.2",
+      "resolved": "https://registry.npmjs.org/@payloadcms/live-preview/-/live-preview-3.85.2.tgz",
+      "integrity": "sha512-nWOQhBv4ei3uEjJGGCo1m541fx4D8wONLv74/yjzsawDHRq6tOcOMUTQuQPIZt5xNlomYFPH9XlZfgIs3BSWdA==",
+      "license": "MIT"
+    },
+    "node_modules/@payloadcms/live-preview-react": {
+      "version": "3.85.2",
+      "resolved": "https://registry.npmjs.org/@payloadcms/live-preview-react/-/live-preview-react-3.85.2.tgz",
+      "integrity": "sha512-HMulvQRXAQp2OaqmXgZa5Sg1htN+5FC36g0vFnoYooG5XtIuRriFNPOPbokygg67AOQPo+0G0r6jPM9JTjYkXA==",
+      "license": "MIT",
+      "dependencies": {
+        "@payloadcms/live-preview": "3.85.2"
+      },
+      "peerDependencies": {
+        "react": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.1 || ^19.1.2 || ^19.2.1",
+        "react-dom": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.1 || ^19.1.2 || ^19.2.1"
+      }
+    },
     "node_modules/@rtsao/scc": {
       "version": "1.1.0",
       "resolved": "https://registry.npmjs.org/@rtsao/scc/-/scc-1.1.0.tgz",