feat: direct messages

1:1 conversations (userAId < userBId dedup pair), messages,
per-participant read tracking (conversation_reads). Block relationship
is enforced on every send. UI: /messages list + /messages/[id] thread
(5s poll), MessageButton on profiles, unread-badged nav icon.

This completes the social-feature backlog: notifications, rate
limiting, blocking, reporting, search/discovery, mentions, DMs, plus
fixes for the recipe-visibility 404, follow race, and 2-level comment
thread cap found during the earlier audit.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/components/layout/nav.tsx

@@ -22,6 +22,7 @@ import {
 } from "@/components/ui/sheet";
 import { Avatar, AvatarFallback, AvatarImage } from "@/components/ui/avatar";
 import { NotificationBell } from "@/components/social/notification-bell";
+import { MessagesNavLink } from "@/components/social/messages-nav-link";
 import { authClient } from "@/lib/auth/client";
 import { useTranslations } from "next-intl";
 
@@ -101,6 +102,7 @@ export function Nav() {
           ))}
         </nav>
         <div className="ml-auto flex items-center gap-2">
+          <MessagesNavLink />
           <NotificationBell />
           <DropdownMenu>
             <DropdownMenuTrigger className="rounded-full outline-none focus-visible:ring-2 focus-visible:ring-ring">