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.

app/components/LiveRichText.tsx

@@ -0,0 +1,25 @@
+"use client";
+
+import { useLivePreview } from "@payloadcms/live-preview-react";
+import { RichText } from "./RichText";
+
+const PAYLOAD_URL = process.env.NEXT_PUBLIC_PAYLOAD_URL || "https://payload.mk360.de";
+
+// Wraps just the RichText body of a legal page (/agb, /datenschutz,
+// /impressum, /widerruf) for Payload Live Preview — those 4 pages' own
+// headings/sidebars/TOC are hardcoded per page, not sourced from
+// `page.title` at all, so `content` is the only field that actually
+// benefits from real-time editing preview. Falls back to the
+// server-fetched `initialContent` until a postMessage arrives, which only
+// happens at all while this page is open inside the Payload admin's Live
+// Preview iframe — ordinary visitors never mount this differently from a
+// plain <RichText>.
+export function LiveRichText({ initialContent, quoteLabel }: { initialContent: unknown; quoteLabel?: string }) {
+  const { data } = useLivePreview<{ content: unknown }>({
+    initialData: { content: initialContent },
+    serverURL: PAYLOAD_URL,
+    depth: 2,
+  });
+
+  return <RichText content={data.content} quoteLabel={quoteLabel} />;
+}