feat: add optimistic UI on AmountInput primary action (Closes #853)

[waterWang]

Summary

Adds optimistic UI feedback to the AmountInput primary action (Continue button). When isSubmitting is true:

• Continue button shows an inline spinner and "Processing…" label via the existing PendingButton component
• All interactive elements are disabled (stepper buttons, number input, Max button, preset chips, Back button)
• Root container gets aria-busy="true" for screen reader announcements
• Disabled styling (disabled:opacity-40 disabled:cursor-not-allowed) applied consistently

The revert-on-failure pattern is handled by the parent component — the parent sets isSubmitting=false on error, which restores the normal Continue button and re-enables all inputs.

Changes

• AmountInput.tsx: Added isSubmitting prop, integrated PendingButton, disabled all interactive elements when submitting
• AmountInput.test.tsx: Added 5 tests covering spinner display, label swap, disabled state, and aria-busy behavior

Closes #853