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/LoginScreen.svelte

@@ -0,0 +1,17 @@
+<script lang="ts">
+	// A full-screen sign-in gate. The anchor is a real navigation to the
+	// server-side /auth/login route (which 302s to Casdoor), not a fetch.
+</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-80 shadow-xl">
+		<div class="card-body items-center gap-4 text-center">
+			<h1 class="flex items-center justify-center gap-2 text-3xl font-bold">
+				<span class="text-orange-500">🔥</span>
+				Hold Slayer
+			</h1>
+			<p class="text-sm opacity-60">Sign in to the gateway</p>
+			<a href="/auth/login" class="btn btn-primary w-full">Sign in with SSO</a>
+		</div>
+	</div>
+</div>