feat(web): flatten admin shell — horizontal tabs, drop redundant banner

The /admin layout had three nested concentric shells: the global Shell
header + main nav, then a per-admin header banner with a Sword icon, then
a left-aligned admin sub-nav (AdminSidebar) — two competing left-edge
navigation columns plus a redundant 'Admin' label that the URL and active
nav already conveyed.

Replaces the side sub-nav with a horizontal tab strip across the top of
the admin content (matches the discover-page tab pattern), drops the
duplicate header banner, and removes the disabled Users/Library
placeholder items so the tab strip only shows surfaces that actually
ship today. The component is renamed AdminSidebar -> AdminTabs to match
its new shape.

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

web/src/lib/components/AdminTabs.svelte

@@ -0,0 +1,38 @@
+<script lang="ts">
+  import { page } from '$app/state';
+
+  type Item = { href: string; label: string };
+
+  // Users and Library are deferred until they have real routes.
+  // Adding them as disabled items clutters the tab strip without serving the
+  // operator — they'll be added back when each surface ships.
+  const items: Item[] = [
+    { href: '/admin',              label: 'Overview' },
+    { href: '/admin/integrations', label: 'Integrations' },
+    { href: '/admin/requests',     label: 'Requests' },
+    { href: '/admin/quarantine',   label: 'Quarantine' }
+  ];
+
+  function isActive(href: string): boolean {
+    if (href === '/admin') return page.url.pathname === '/admin';
+    return page.url.pathname.startsWith(href);
+  }
+</script>
+
+<nav aria-label="Admin sections" class="border-b border-border">
+  <ul class="flex gap-2">
+    {#each items as item (item.href)}
+      <li>
+        <a
+          href={item.href}
+          aria-current={isActive(item.href) ? 'page' : undefined}
+          class="block border-b-2 px-3 py-2 text-sm transition-colors {isActive(item.href)
+            ? 'border-accent text-text-primary'
+            : 'border-transparent text-text-secondary hover:text-text-primary'}"
+        >
+          {item.label}
+        </a>
+      </li>
+    {/each}
+  </ul>
+</nav>