Email-first membership: signup/login, wallet linked at purchase time

Normal people join with email + password (sponsor attribution via cookie at
signup); the wallet only appears when buying or activating payouts, and gets
linked to the account then. Wallet-only sign-in remains for crypto-native
users. Sessions carry {email, address, memberId}.

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

public/assets/home.js

@@ -31,6 +31,12 @@
   async function buyPack(btn) {
     try {
       btn.disabled = true;
+      // email members get their wallet linked to the account at buy time
+      const me = await (await fetch('/api/me')).json();
+      if (me.signedIn && me.email && !me.address) {
+        IAP.status('First, a free signature links your wallet to your account…');
+        await IAPWallet.signIn();
+      }
       IAP.status('Confirm the purchase in your wallet…');
       const r = await IAPWallet.buy(Number(btn.dataset.id), sp.sponsorId || 0, btn.dataset.cost);
       if (r.receipt.status !== '0x1') throw new Error('Transaction reverted. Check the explorer.');