Migrate user/profile URLs to /u/[handle] and remove legacy scripts

All user and post profile URLs have been updated from /[handle] to /u/[handle] for consistency. Added a redirect page for /posts/[id] to the canonical /u/[handle]/posts/[id] route. Deprecated and removed legacy scripts and documentation related to chat and federation endpoints. Push-based federation endpoints are now disabled in favor of real-time pull-based federation.

src/app/explore/page.tsx

@@ -22,7 +22,7 @@ interface User {
 
 function UserCard({ user }: { user: User }) {
     return (
-        <Link href={`/${user.handle}`} className="user-card">
+        <Link href={`/u/${user.handle}`} className="user-card">
             <div className="avatar">
                 {user.avatarUrl ? (
                     <img src={user.avatarUrl} alt={user.displayName} />