feat: self-service user signup (#672)

Co-authored-by: Elias Schneider <login@eliasschneider.com>

frontend/src/routes/st/[token]/+page.ts

@@ -0,0 +1,16 @@
+import { redirect } from '@sveltejs/kit';
+import type { PageLoad } from './$types';
+
+// Alias for /signup?token=...
+export const load: PageLoad = async ({ url, params }) => {
+	const targetPath = '/signup';
+
+	const searchParams = new URLSearchParams();
+	searchParams.set('token', params.token);
+
+	if (url.searchParams.has('redirect')) {
+		searchParams.set('redirect', url.searchParams.get('redirect')!);
+	}
+
+	return redirect(307, `${targetPath}?${searchParams.toString()}`);
+};