feat(28-03): add profile navigation link and extend /me with createdAt

Adds Profile link to UserMenu dropdown (above Settings), extends /me
endpoint to return user's createdAt for member-since display, and
updates AuthState interface with optional createdAt field.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/client/hooks/useAuth.ts

@@ -2,7 +2,7 @@ import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { apiDelete, apiGet, apiPost } from "../lib/api";
 
 interface AuthState {
-	user: { id: string; email?: string } | null;
+	user: { id: string; email?: string; createdAt?: string } | null;
 	authenticated: boolean;
 }