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/GetPaid.tsx

@@ -267,7 +267,7 @@ export default function GetPaid({
       )}
 
       {isOpen && (
-        <div className="fixed inset-0 bg-black bg-opacity-50 flex items-center justify-center z-50 p-4">
+        <div className="fixed inset-0 bg-black/50 flex items-center justify-center z-50 p-4">
           <div className="bg-white rounded-lg p-6 max-w-2xl w-full max-h-[90vh] overflow-y-auto">
             <h3 className="text-xl font-bold text-gray-900 mb-4">Record Paycheck</h3>
 
@@ -524,7 +524,7 @@ export default function GetPaid({
       )}
 
       {showPreview && (
-        <div className="fixed inset-0 bg-black bg-opacity-50 flex items-center justify-center z-50 p-4">
+        <div className="fixed inset-0 bg-black/50 flex items-center justify-center z-50 p-4">
           <div className="bg-white rounded-lg p-6 max-w-2xl w-full max-h-[90vh] overflow-y-auto">
             <h3 className="text-xl font-bold text-gray-900 mb-4">Confirm Paycheck Allocation</h3>