Replace sponsor program with $VOICEBOX token

- Remove the sponsor feature (sponsors page, homepage promo, footer
  link, Stripe constants, and the app About-page sponsored-by section)
- Add $VOICEBOX token: navbar pill linking to pump.fun and a footer
  Token column with a copyable Solana contract address
- Drop API and Download from the navbar links

landing/src/components/CopyAddress.tsx

@@ -0,0 +1,38 @@
+'use client';
+
+import { Check, Copy } from 'lucide-react';
+import { useState } from 'react';
+
+/** Compact, copyable contract address — short display, full value to clipboard. */
+export function CopyAddress({ address }: { address: string }) {
+  const [copied, setCopied] = useState(false);
+
+  const handleCopy = async () => {
+    try {
+      await navigator.clipboard.writeText(address);
+      setCopied(true);
+      setTimeout(() => setCopied(false), 2000);
+    } catch {
+      // Clipboard unavailable (e.g. insecure context) — silently no-op.
+    }
+  };
+
+  const short = `${address.slice(0, 4)}…${address.slice(-4)}`;
+
+  return (
+    <button
+      type="button"
+      onClick={handleCopy}
+      title={address}
+      aria-label={copied ? 'Contract address copied' : 'Copy contract address'}
+      className="group inline-flex items-center gap-2 rounded-lg border border-border/60 bg-card/60 px-2.5 py-1.5 font-mono text-xs text-muted-foreground transition-colors hover:border-border hover:text-foreground"
+    >
+      <span>{short}</span>
+      {copied ? (
+        <Check className="h-3.5 w-3.5 text-accent" />
+      ) : (
+        <Copy className="h-3.5 w-3.5 opacity-60 transition-opacity group-hover:opacity-100" />
+      )}
+    </button>
+  );
+}