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/social/favorite-button.tsx

@@ -0,0 +1,38 @@
+"use client";
+
+import { useState } from "react";
+import { Heart } from "lucide-react";
+import { toast } from "sonner";
+import { Button } from "@/components/ui/button";
+import { cn } from "@/lib/utils";
+
+export function FavoriteButton({
+  recipeId,
+  initialFavorited = false,
+}: {
+  recipeId: string;
+  initialFavorited?: boolean;
+}) {
+  const [favorited, setFavorited] = useState(initialFavorited);
+  const [loading, setLoading] = useState(false);
+
+  async function toggle() {
+    setLoading(true);
+    try {
+      const res = await fetch(`/api/v1/recipes/${recipeId}/favorite`, {
+        method: favorited ? "DELETE" : "POST",
+      });
+      if (!res.ok) return;
+      setFavorited(!favorited);
+    } finally {
+      setLoading(false);
+    }
+  }
+
+  return (
+    <Button variant="ghost" size="sm" onClick={toggle} disabled={loading} className="gap-1.5">
+      <Heart className={cn("h-4 w-4", favorited && "fill-red-500 text-red-500")} />
+      {favorited ? "Saved" : "Save"}
+    </Button>
+  );
+}