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/theme/index.ts

@@ -783,6 +783,32 @@ export const theme = createTheme({
         },
       },
     },
+    MuiCheckbox: {
+      styleOverrides: {
+        root: {
+          color: t.ColorNeutral400,
+          transition: 'color 150ms ease-in-out',
+          '&:hover': {
+            color: t.ColorNeutral600,
+            backgroundColor: 'transparent',
+          },
+          '&.Mui-checked': {
+            color: t.ColorInteractiveDefault,
+            '&:hover': {
+              color: t.ColorInteractiveHover,
+            },
+          },
+          '&:focus-visible': {
+            outline: `2px solid ${t.ColorInteractiveFocus}`,
+            outlineOffset: '2px',
+            borderRadius: '4px',
+          },
+          '&.Mui-disabled': {
+            color: t.ColorNeutral300,
+          },
+        },
+      },
+    },
     MuiRadio: {
       styleOverrides: {
         root: {