feat: add high-contrast mode overrides for AmountInput (Closes #752)

[waterWang]

Summary

Adds high-contrast mode (prefers-contrast: more) overrides for the AmountInput component (Closes #752). When a user's OS is set to "more contrast", the input field and status chips get explicit solid borders and full-contrast text colours.

Changes

src/pages/AmountInput.tsx

• Added amount-input wrapper class to the root container for scoped high-contrast CSS targeting.
• Added import of ../styles/contrast.css.

src/styles/contrast.css (new)

• Added a new @media (prefers-contrast: more) section scoped to .amount-input:
  • Forces solid black border and white background on the input field.
  • Ensures placeholder text is fully opaque.
  • Strengthens status chip borders to 1.5px solid black.
  • Promotes muted text to high-contrast equivalents.
  • Forces all card/background/muted utilities to solid white.

src/pages/__tests__/AmountInput.contrast.test.tsx (new)

• Verifies the amount-input wrapper class is applied.
• Verifies the component imports ../styles/contrast.css.
• Verifies status badges (success/warning/error) are rendered with the rounded-sm class that the contrast overrides depend on.
• Verifies the input field is rendered as a focusable native element.

Verification

• All 6 tests pass for the new contrast test suite.

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