feat(ui): add OverflowPillDisplay — non-interactive aria-hidden +N span

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/OverflowPillDisplay.svelte

@@ -0,0 +1,14 @@
+<script lang="ts">
+type Props = {
+	extraCount: number;
+};
+
+let { extraCount }: Props = $props();
+</script>
+
+<span
+	aria-hidden="true"
+	class="inline-flex shrink-0 items-center rounded-full border border-line bg-muted px-2 py-0.5 text-[9px] font-bold text-ink-2"
+>
+	+{extraCount}
+</span>