Restructure /admin into tabbed area with password gate

Backups moves under /admin/backups; new Reports and Categories tabs join
it (categories migrated from the top-level /categories route). The
dashboard's SKU/low-stock/inventory-value cards move into Reports, which
also adds sales totals and a top-selling parts list.

A 5-minute sliding-cookie password gate (27182818) now wraps every
/admin request, including the backup download endpoint, via a
hooks.server.js auth check. The login page sits at /admin/login and
escapes the admin tab chrome via a layout reset.

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

src/routes/admin/+layout.svelte

@@ -0,0 +1,61 @@
+<script>
+  import { page } from '$app/stores';
+  import { t } from '$lib/i18n/store.js';
+
+  const tabs = [
+    { href: '/admin/reports',    key: 'admin.tabs.reports' },
+    { href: '/admin/categories', key: 'admin.tabs.categories' },
+    { href: '/admin/backups',    key: 'admin.tabs.backups' }
+  ];
+
+  $: current = $page.url.pathname;
+  function isActive(href) {
+    return current === href || current.startsWith(href + '/');
+  }
+</script>
+
+<h1>{$t('admin.title')}</h1>
+
+<nav class="tabs" aria-label="Admin sections">
+  {#each tabs as tab}
+    <a href={tab.href} class:active={isActive(tab.href)} aria-current={isActive(tab.href) ? 'page' : undefined}>
+      {$t(tab.key)}
+    </a>
+  {/each}
+</nav>
+
+<div class="tab-panel">
+  <slot />
+</div>
+
+<style>
+  h1 { margin-bottom: 0.75rem; }
+  .tabs {
+    display: flex;
+    gap: 0.25rem;
+    border-bottom: 2px solid #e5e8ee;
+    margin-bottom: 1.5rem;
+    flex-wrap: wrap;
+  }
+  .tabs a {
+    padding: 0.55rem 1rem;
+    text-decoration: none;
+    color: #1d2330;
+    border: 1px solid transparent;
+    border-bottom: none;
+    border-radius: 6px 6px 0 0;
+    margin-bottom: -2px;
+    font-weight: 500;
+    background: transparent;
+  }
+  .tabs a:hover {
+    background: #eef1f6;
+    color: #1d2330;
+  }
+  .tabs a.active {
+    background: #fff;
+    border-color: #e5e8ee;
+    border-bottom-color: #fff;
+    color: #006a4e;
+  }
+</style>