agent: components — add shared EmptyState component

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/components/EmptyState.svelte

@@ -0,0 +1,17 @@
+<script>
+  let { icon: Icon = null, message = '', actionLabel = '', onAction = null } = $props();
+</script>
+
+<div class="flex flex-col items-center justify-center gap-4 py-16 text-text-secondary">
+  {#if Icon}
+    <Icon size={48} strokeWidth={1} class="opacity-40" aria-hidden="true" />
+  {/if}
+  <p class="text-center max-w-xs leading-relaxed">{message}</p>
+  {#if actionLabel && onAction}
+    <button onclick={onAction}
+      class="px-4 py-2 rounded-lg bg-accent text-bg text-sm font-medium hover:bg-accent-hover"
+      style="transition-duration: var(--duration-ui)">
+      {actionLabel}
+    </button>
+  {/if}
+</div>