docs: add Claude AI assistant rules and configuration

Add comprehensive rule documentation for AI-assisted development covering
authentication surfaces, outbound-call safety invariants, and other project
conventions to guide Claude's understanding of critical system behaviors.

dashboard/src/lib/components/DeniedScreen.svelte

@@ -0,0 +1,17 @@
+<script lang="ts">
+	import { auth } from '$lib/auth.svelte';
+</script>
+
+<div class="bg-base-100 fixed inset-0 z-[9999] flex items-center justify-center p-4">
+	<div class="card bg-base-200 w-96 shadow-xl">
+		<div class="card-body items-center gap-4 text-center">
+			<h1 class="text-2xl font-bold">Not authorized</h1>
+			<p class="text-sm opacity-70">
+				Hold Slayer is a single-operator gateway. You're signed in as
+				<span class="font-medium">{auth.user?.display_name ?? auth.user?.name}</span>,
+				but this gateway is reserved for its owner.
+			</p>
+			<a href="/auth/logout" class="btn btn-outline w-full">Sign out</a>
+		</div>
+	</div>
+</div>