Checkbox atom: new FA wrapper with brand theming

- New Checkbox atom wrapping MUI Checkbox (forwardRef, displayName)
- MuiCheckbox theme overrides: warm gold checked, focus ring, disabled muted
- Stories: Default, States, TermsAgreement, Checklist
- PaymentStep: now imports Checkbox from atom instead of MUI directly

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/pages/PaymentStep/PaymentStep.tsx

@@ -1,7 +1,7 @@
 import React from 'react';
 import Box from '@mui/material/Box';
 import Paper from '@mui/material/Paper';
-import Checkbox from '@mui/material/Checkbox';
+import { Checkbox } from '../../atoms/Checkbox';
 import FormControlLabel from '@mui/material/FormControlLabel';
 import type { SxProps, Theme } from '@mui/material/styles';
 import LockOutlinedIcon from '@mui/icons-material/LockOutlined';