add shared computer feature

frontend/src/routes/[fam]/switch/+page.server.ts

@@ -0,0 +1,13 @@
+import { redirect } from '@sveltejs/kit';
+
+// The fam layout renders the shared-device picker when child sessions exist
+// but none is active. With an active session this route just sends you home.
+export async function load(event) {
+	const session = event.locals.user;
+	if (session) {
+		const fam = event.params.fam;
+		if (session.role === 'parent') throw redirect(303, `/${fam}`);
+		throw redirect(303, `/${fam}/${encodeURIComponent(session.username || '')}`);
+	}
+	return {};
+}

frontend/src/routes/[fam]/switch/+page.svelte

@@ -0,0 +1,4 @@
+<!-- Shared-device profile picker. The fam layout renders the picker itself in
+     picker mode (no active session, child sessions present) — this page only
+     exists so /{fam}/switch resolves as a route. -->
+<div></div>