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/follow-button.tsx

@@ -0,0 +1,42 @@
+"use client";
+
+import { useState } from "react";
+import { toast } from "sonner";
+import { Button } from "@/components/ui/button";
+
+export function FollowButton({
+  targetUserId: _targetUserId,
+  targetUsername,
+  initialFollowing = false,
+}: {
+  targetUserId: string;
+  targetUsername: string;
+  initialFollowing?: boolean;
+}) {
+  const [following, setFollowing] = useState(initialFollowing);
+  const [loading, setLoading] = useState(false);
+
+  async function toggle() {
+    setLoading(true);
+    try {
+      const res = await fetch(`/api/v1/users/${targetUsername}/follow`, {
+        method: following ? "DELETE" : "POST",
+      });
+      if (!res.ok) {
+        const err = await res.json() as { error?: string };
+        toast.error(err.error ?? "Failed");
+        return;
+      }
+      setFollowing(!following);
+      toast.success(following ? "Unfollowed" : "Following");
+    } finally {
+      setLoading(false);
+    }
+  }
+
+  return (
+    <Button variant={following ? "outline" : "default"} size="sm" onClick={toggle} disabled={loading}>
+      {loading ? "…" : following ? "Following" : "Follow"}
+    </Button>
+  );
+}