feat(social): follows, favorites, comments, reactions, collections, public profiles

Follow/unfollow users. Recipe favorites. Threaded comments with emoji reactions.
Collections (public/private) with shared member invite. Activity feed.
Public profile pages at /u/[username].

apps/web/components/collections/fork-collection-button.tsx

@@ -0,0 +1,36 @@
+"use client";
+
+import { useState } from "react";
+import { useRouter } from "next/navigation";
+import { GitFork } from "lucide-react";
+import { Button } from "@/components/ui/button";
+import { toast } from "sonner";
+
+export function ForkCollectionButton({ collectionId }: { collectionId: string }) {
+  const [forking, setForking] = useState(false);
+  const router = useRouter();
+
+  async function handleFork() {
+    setForking(true);
+    try {
+      const res = await fetch(`/api/v1/collections/${collectionId}/fork`, { method: "POST" });
+      if (!res.ok) {
+        const data = await res.json() as { error?: string };
+        throw new Error(data.error ?? "Fork failed");
+      }
+      const { id } = await res.json() as { id: string };
+      toast.success("Collection forked to your library");
+      router.push(`/collections/${id}`);
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : "Fork failed");
+      setForking(false);
+    }
+  }
+
+  return (
+    <Button variant="outline" size="sm" onClick={() => { void handleFork(); }} disabled={forking} className="gap-2 shrink-0">
+      <GitFork className="h-4 w-4" />
+      {forking ? "Forking…" : "Fork collection"}
+    </Button>
+  );
+}