Fix modal overlays — replace deprecated Tailwind v4 bg-opacity syntax

All 11 modal backdrop divs used `bg-black bg-opacity-50` which Tailwind v4
dropped, causing solid-black overlays. Replaced with `bg-black/50` across
AuthModal, AccountManagement, EnvelopeCreate, EnvelopeEdit, GetPaid,
SetupWizard, TransactionEdit, and page.tsx.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/TransactionEdit.tsx

@@ -49,7 +49,7 @@ export default function TransactionEdit({ transaction, envelopes, accounts, onSa
   };
 
   return (
-    <div className="fixed inset-0 bg-black bg-opacity-50 flex items-center justify-center z-50">
+    <div className="fixed inset-0 bg-black/50 flex items-center justify-center z-50">
       <div className="bg-white rounded-lg p-6 w-full max-w-md mx-4">
         <h3 className="text-lg font-semibold text-gray-900 mb-4">Edit Transaction</h3>