feat: add payment success state UI

[manueldezman]

Closes #52

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Summary

Implements a paid-state success experience for the /pay flow so users get clear, reassuring feedback after confirming payment.

Changes

• Added a new PaymentSuccess component with:
  • Green success checkmark treatment
  • Heading text: Payment Successful!
  • Reassuring success copy
  • Dummy Download Receipt button
• Updated PayTabs to:
  • Track local paid state
  • Trigger success state after manual-transfer confirmation + short mock verification delay
  • Replace the payment tabs/form with the success component when paid
• Kept connected-wallet behavior unchanged (still mock “wallet connected” flow)
• Fixed unescaped JSX text in manual transfer helper copy to satisfy linting
• Updated success paragraph text color to white per UI preference

Verification

• npm run typecheck
• npm run format:check
• npm run lint

Notes

This success state is intentionally local UI state only; refreshing /pay returns users to the payment form.

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