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/app/page.tsx

@@ -1153,7 +1153,7 @@ export default function BudgetDashboard() {
       {showAuthModal && <AuthModal isOpen={showAuthModal} onClose={() => setShowAuthModal(false)} onAuthSuccess={handleAuthSuccess} />}
       {showSetupWizard && <SetupWizard onComplete={handleSetupComplete} onSkip={() => setShowSetupWizard(false)} userId={user?.userId || ''} />}
       {showGetPaidModal && (
-        <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 onClick={(e) => e.stopPropagation()} className="w-full">
             <GetPaid
               accounts={accounts}