fix login confusion and enforce reactive subscription after login

frontend/src/routes/join/+page.server.ts

@@ -0,0 +1,34 @@
+import { fail, redirect } from '@sveltejs/kit';
+import { redeemOtp } from '$lib/server/member-otp';
+import { slugify, handle } from '@shared/slugify';
+import { setSessionCookie, clearLegacyCookies } from '$lib/server/session';
+
+export const actions = {
+	default: async (event) => {
+		const fd = await event.request.formData();
+		const famName = (fd.get('family') || '').toString().trim();
+		const name = (fd.get('name') || '').toString().trim();
+		const otp = (fd.get('otp') || '').toString().trim();
+
+		if (!famName) return fail(400, { error: 'Enter your family name.', famName, name, otp });
+		if (!name) return fail(400, { error: 'Enter your name.', famName, name, otp });
+		if (!otp) return fail(400, { error: 'Enter the code shown by your parent.', famName, name, otp });
+
+		const famSlug = slugify(famName);
+		try {
+			const token = await redeemOtp({ famSlug, username: name, otp });
+			clearLegacyCookies(event.cookies);
+			setSessionCookie(event.cookies, token);
+		} catch (e) {
+			return fail(400, {
+				error: e instanceof Error ? e.message : 'Join failed',
+				famName,
+				name,
+				otp
+			});
+		}
+
+		const handleName = handle(name);
+		throw redirect(303, `/${famSlug}/${encodeURIComponent(handleName)}`);
+	}
+};