fix: Svelte 5 runes mode 호환 ($: → $derived/$effect)

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

frontend/src/routes/+layout.svelte

@@ -20,14 +20,14 @@
     authChecked = true;
   });
 
-  $: {
+  $effect(() => {
     if (browser && authChecked && !$isAuthenticated && !PUBLIC_PATHS.some(p => $page.url.pathname.startsWith(p))) {
       goto('/login');
     }
-  }
+  });
 
   // 사이드바 표시 여부
-  $: showSidebar = $isAuthenticated && !NO_SIDEBAR_PATHS.some(p => $page.url.pathname.startsWith(p));
+  let showSidebar = $derived($isAuthenticated && !NO_SIDEBAR_PATHS.some(p => $page.url.pathname.startsWith(p)));
 
   // 키보드 단축키
   function handleKeydown(e) {