fix: guard goto() with browser check to prevent SSR crash

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/routes/+layout.svelte

@@ -1,5 +1,6 @@
 <script>
   import { onMount } from 'svelte';
+  import { browser } from '$app/environment';
   import { page } from '$app/stores';
   import { goto } from '$app/navigation';
   import { isAuthenticated, tryRefresh, logout } from '$lib/stores/auth';
@@ -15,7 +16,7 @@
   });
 
   $: {
-    if (!$isAuthenticated && !PUBLIC_PATHS.some(p => $page.url.pathname.startsWith(p))) {
+    if (browser && !$isAuthenticated && !PUBLIC_PATHS.some(p => $page.url.pathname.startsWith(p))) {
       goto('/login');
     }
   }