feat(web): add /admin layout with role-gated load + sidebar

Hard route gate in admin/+layout.ts redirects non-admins to / before
the layout (or any child) renders. AdminSidebar reads the active route
from $app/state and applies a 12% accent-tinted bg + 2px forest-teal
left strip on the active item. Overview landing shows pending-request
count and Lidarr connected/unset, each linking to its sub-page. Shell
nav exposes the Admin link only to is_admin users.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/src/routes/admin/+page.svelte

@@ -0,0 +1,37 @@
+<script lang="ts">
+  import { createAdminRequestsQuery, createLidarrConfigQuery } from '$lib/api/admin';
+
+  const requestsStore = createAdminRequestsQuery('pending');
+  const requests = $derived($requestsStore);
+
+  const configStore = createLidarrConfigQuery();
+  const config = $derived($configStore);
+
+  const pendingCount = $derived(requests.data?.length ?? 0);
+  const lidarrConnected = $derived(config.data?.enabled === true);
+</script>
+
+<div class="space-y-6">
+  <header>
+    <h2 class="font-display text-2xl font-medium text-text-primary">Overview</h2>
+  </header>
+
+  <div class="grid gap-4 sm:grid-cols-2">
+    <a
+      href="/admin/requests"
+      class="block rounded-xl border border-border bg-surface p-5 transition-colors hover:bg-surface-hover"
+    >
+      <div class="text-sm text-text-secondary">Pending requests</div>
+      <div class="font-display mt-1 text-3xl font-medium text-text-primary">{pendingCount}</div>
+    </a>
+    <a
+      href="/admin/integrations"
+      class="block rounded-xl border border-border bg-surface p-5 transition-colors hover:bg-surface-hover"
+    >
+      <div class="text-sm text-text-secondary">Lidarr</div>
+      <div class="font-display mt-1 text-2xl font-medium text-text-primary">
+        {lidarrConnected ? 'Connected' : 'Unset'}
+      </div>
+    </a>
+  </div>
+</div>