fix: harden booking and meeting flows

app/(payload)/api/booking/slots/route.ts

@@ -2,6 +2,7 @@ import { NextResponse } from "next/server";
 import { getPayload } from "payload";
 import config from "@payload-config";
 import { getAvailableSlots } from "@/lib/booking/slots";
+import { berlinDayBounds, parseBerlinDate } from "@/lib/calendar/timezone";
 
 export const dynamic = "force-dynamic";
 
@@ -29,16 +30,15 @@ export async function GET(request: Request) {
       return NextResponse.json({ error: "Dieses Angebot ist nicht buchbar." }, { status: 404 });
     }
 
-    const from = fromParam ? new Date(`${fromParam}T00:00:00`) : new Date();
-    if (Number.isNaN(from.getTime())) {
+    const from = fromParam ? parseBerlinDate(fromParam) : berlinDayBounds(new Date()).start;
+    if (!from) {
       return NextResponse.json({ error: "Ungültiges Startdatum." }, { status: 400 });
     }
-    from.setHours(0, 0, 0, 0);
-    const to = toParam ? new Date(`${toParam}T00:00:00`) : new Date(from.getTime() + 21 * 24 * 60 * 60 * 1000);
-    if (Number.isNaN(to.getTime())) {
+    const toDate = toParam ? parseBerlinDate(toParam) : new Date(from.getTime() + 21 * 24 * 60 * 60 * 1000);
+    if (!toDate) {
       return NextResponse.json({ error: "Ungültiges Enddatum." }, { status: 400 });
     }
-    to.setHours(23, 59, 59, 999);
+    const to = berlinDayBounds(toDate).end;
 
     if (to < from) {
       return NextResponse.json({ error: "Das Enddatum muss nach dem Startdatum liegen." }, { status: 400 });