feat: show unseen count in macos dock

Resolves #93

js/src/hooks/use-dock-badge.ts

@@ -0,0 +1,27 @@
+import { useEffect, useMemo } from "react";
+import type { Particle, StreamProperties } from "@/api/types";
+
+type StreamParticle = Particle & { type: "stream"; properties: StreamProperties };
+
+/**
+ * Syncs the macOS dock badge with the count of unseen streams.
+ * Both last_child_created_at and playback_markers store the child particle's
+ * created_at, so they're directly comparable.
+ */
+export function useDockBadge(streams: StreamParticle[], userId: string | undefined) {
+  const unseenCount = useMemo(() => {
+    if (!userId) return 0;
+    return streams.filter((s) => {
+      const lastActivity = s.last_child_created_at?.getTime();
+      if (!lastActivity) return false;
+      const marker = s.playback_markers?.[userId]?.getTime();
+      if (marker === undefined) return false;
+      return lastActivity > marker;
+    }).length;
+  }, [streams, userId]);
+
+  useEffect(() => {
+    window.electronApp.setDockBadge(unseenCount);
+    return () => window.electronApp.setDockBadge(0);
+  }, [unseenCount]);
+}