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.test.ts

@@ -42,7 +42,7 @@ describe('auth guard (hooks.server.ts handle)', () => {
 		expect(resolve).toHaveBeenCalledWith(event);
 	});
 
-	it.each(['/login', '/login/', '/register', '/invite/abc123'])(
+	it.each(['/login', '/login/', '/register', '/signup', '/signup/', '/invite/abc123'])(
 		'allows public route %s without auth',
 		async (path) => {
 			const { event, resolve } = createEvent(path);

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'];