Add calendar-based booking system: accounts, availability, admin calendar

- Customer accounts (separate auth collection) with /konto area
- Availability + AvailabilityOverrides collections driving real slot calculation
- BookingRequests with race-safe confirmation (Postgres advisory lock + transaction)
- Booking emails (request received, admin notify, confirmed, rejected, alternative proposed/accepted, cancelled)
- Confirmed online bookings auto-create a private linked video-call Event
- Custom Payload admin calendar view (month grid + day schedule)
- Wired booking widget into offer pages and /termin-buchen

components/admin/CalendarNavLink.tsx

@@ -0,0 +1,23 @@
+import Link from "next/link";
+
+export function CalendarNavLink() {
+  return (
+    <div style={{ padding: "0 8px 8px" }}>
+      <Link
+        href="/admin/kalender"
+        style={{
+          display: "block",
+          padding: "8px 12px",
+          borderRadius: 6,
+          fontSize: 13,
+          fontWeight: 600,
+          color: "#8b616d",
+          textDecoration: "none",
+          border: "1px solid #e2e2e2",
+        }}
+      >
+        📅 Kalender
+      </Link>
+    </div>
+  );
+}