feat(auth): add /signup to public routes

Allow unauthenticated access to the signup page.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/hooks.server.ts

@@ -2,7 +2,7 @@ import type { Handle } from '@sveltejs/kit';
 import { redirect } from '@sveltejs/kit';
 import { apiClient } from '$lib/server/api';
 
-const PUBLIC_ROUTES = ['/login', '/register', '/invite'];
+const PUBLIC_ROUTES = ['/login', '/register', '/signup', '/invite'];
 
 const STATIC_PREFIXES = ['/_app/', '/favicon'];