Unlock everything, add a donation link, credit Last.fm, nudge re-linking

- Every feature is free for everyone (the plan system stays for later);
  remaining limits are abuse guards
- Optional website donation link (NEXT_PUBLIC_DONATION_URL). Not shown in
  the mobile apps, where tips to the developer generally need in-app
  purchase
- Last.fm/ListenBrainz-sourced plays link to the user's profile, as
  Last.fm's API terms require
- Connections track when they last saw activity; a scrobbler silent for
  over a week prompts re-linking Spotify (its link expires every 6 months)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MzFywujuH9TiuXbr9vjoX8

apps/web/src/components/Nav.tsx

@@ -3,6 +3,7 @@
 import Link from 'next/link';
 import { useRouter } from 'next/navigation';
 import { api, useMe } from '@/lib/api';
+import { SupportLink } from './SupportLink';
 
 export function Nav() {
   const { me, mutate } = useMe();
@@ -18,6 +19,7 @@ export function Nav() {
         musicshare
       </Link>
       <div className="nav-links">
+        <SupportLink />
         {me ? (
           <>
             <Link href="/">Feed</Link>