D034: Unify form error styling to copper across theme

- MuiOutlinedInput: error border and focus ring now use copper
  (ColorTextBrand #B0610F) instead of red (ColorFeedbackError #BC2F2F)
- MuiFormHelperText: error text colour overridden to copper
- MuiFormLabel: error state stays neutral text (D024 reinforcement)
- ToggleButtonGroup: error border changed from error.main to copper

palette.error.main remains red for non-form uses (destructive buttons,
system alerts). Form validation is now fully warm/copper end-to-end.

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

src/components/atoms/ToggleButtonGroup/ToggleButtonGroup.tsx

@@ -180,9 +180,9 @@ export const ToggleButtonGroup = React.forwardRef<HTMLFieldSetElement, ToggleBut
                   },
                 },
 
-                // Error border
+                // Error border — copper, not red (D034)
                 ...(error && {
-                  borderColor: 'error.main',
+                  borderColor: 'var(--fa-color-text-brand)',
                   '&.Mui-selected': {
                     borderColor: 'primary.main',
                   },