feat: add TransactionPendingModal component (#9)

[Anuoluwapo25]

Summary

Closes #9

• Adds TransactionPendingModal at src/components/common/TransactionPendingModal.tsx
• Built on the existing Radix UI Dialog shell — opens/closes cleanly via isOpen / onClose props
• Supports three status states: pending (animated spinner), success (check icon), error (X icon)
• Exposes title, statusText, description for flexible copy per flow (buy, sell, etc.)
• Optional actionLabel + onAction for a primary CTA in the action area
• Close button is suppressed during pending state to prevent accidental dismissal
• Matches the dark theme (slate-950 background, amber accents) used throughout the app

Test plan

• Import TransactionPendingModal and toggle isOpen — modal opens and closes cleanly
• Verify pending state: spinner animates, close button is hidden, pulse dot visible
• Verify success state: green check icon, close button appears
• Verify error state: red X icon, close button appears
• Pass actionLabel + onAction — confirm CTA button renders and fires callback
• Confirm modal is dismissible via overlay click or Escape key (Radix default)

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