feat: add WebAuthn related-origins endpoint for cross-domain passkeys

Allows passkeys registered with RP ID rspace.online to work on
rcal.online subdomains via the .well-known/webauthn route.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/app/.well-known/webauthn/route.ts

@@ -0,0 +1,13 @@
+import { NextResponse } from 'next/server';
+
+/**
+ * Related Origin Requests for WebAuthn
+ * Allows passkeys registered with RP ID "rspace.online" to be used on this domain
+ * See: https://passkeys.dev/docs/advanced/related-origins/
+ */
+export async function GET() {
+  return NextResponse.json({
+    origins: ['https://rspace.online'],
+  });
+}
+