feat(ui): add admin area (domains, profiles, backup) with gear link in header

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/routes/admin/+layout.svelte

@@ -0,0 +1,58 @@
+<script lang="ts">
+  import { page } from '$app/stores';
+
+  let { children } = $props();
+
+  const items = [
+    { href: '/admin/domains', label: '🌐 Domains' },
+    { href: '/admin/profiles', label: '👥 Profile' },
+    { href: '/admin/backup', label: '💾 Backup' }
+  ];
+</script>
+
+<nav class="tabs">
+  {#each items as item (item.href)}
+    <a
+      href={item.href}
+      class="tab"
+      class:active={$page.url.pathname.startsWith(item.href)}
+    >
+      {item.label}
+    </a>
+  {/each}
+</nav>
+
+<section class="admin-body">
+  {@render children()}
+</section>
+
+<style>
+  .tabs {
+    display: flex;
+    gap: 0.25rem;
+    padding: 0.75rem 0 0;
+    overflow-x: auto;
+    -webkit-overflow-scrolling: touch;
+  }
+  .tab {
+    padding: 0.6rem 0.9rem;
+    background: white;
+    border: 1px solid #e4eae7;
+    border-radius: 999px;
+    text-decoration: none;
+    color: #444;
+    font-size: 0.95rem;
+    white-space: nowrap;
+    min-height: 40px;
+    display: inline-flex;
+    align-items: center;
+  }
+  .tab.active {
+    background: #2b6a3d;
+    color: white;
+    border-color: #2b6a3d;
+  }
+  .admin-body {
+    padding: 1rem 0;
+  }
+</style>