fix: wait for auth refresh check before redirecting to login

The $: reactive statement was firing before onMount's tryRefresh()
completed, immediately redirecting to /login on every page refresh.
Added authChecked flag to gate the redirect logic.

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

frontend/src/routes/+layout.svelte

@@ -8,15 +8,17 @@
   import '../app.css';
 
   const PUBLIC_PATHS = ['/login', '/setup'];
+  let authChecked = false;
 
   onMount(async () => {
     if (!$isAuthenticated) {
       await tryRefresh();
     }
+    authChecked = true;
   });
 
   $: {
-    if (browser && !$isAuthenticated && !PUBLIC_PATHS.some(p => $page.url.pathname.startsWith(p))) {
+    if (browser && authChecked && !$isAuthenticated && !PUBLIC_PATHS.some(p => $page.url.pathname.startsWith(p))) {
       goto('/login');
     }
   }
@@ -32,7 +34,11 @@
 
 <svelte:window on:keydown={handleKeydown} />
 
-{#if $isAuthenticated || PUBLIC_PATHS.some(p => $page.url.pathname.startsWith(p))}
+{#if !authChecked}
+  <div class="min-h-screen flex items-center justify-center">
+    <p class="text-[var(--text-dim)]">로딩 중...</p>
+  </div>
+{:else if $isAuthenticated || PUBLIC_PATHS.some(p => $page.url.pathname.startsWith(p))}
   <slot />
 {/if}