feat(ui): engineer QR code expiration and staleness visualization

[0dillon]

Summary

SEP-0007 payment URIs embed an exchange rate that is only cryptographically valid for a narrow window (e.g., 5 minutes). Currently, the QR code remains on screen indefinitely, leading customers to scan expired codes and triggering confusing payment failures on the backend.

We must implement a highly visible, real-time expiration visualizer. This involves a shrinking progress bar anchored below the QR code. Once the TTL (Time To Live) expires, the QR code must be aggressively hidden and replaced with an overlay prompting the merchant to 'Tap to Refresh Rate'. The StalenessIndicator component must also be refactored to eliminate hydration mismatch errors.

Acceptance Criteria

• Build a deterministic countdown timer synchronized with the rate TTL.
• Implement a shrinking progress bar component to visualize remaining time.
• Replace the QR code with a refresh overlay exactly when the timer hits zero.
• Fix all server/client hydration errors related to Date.now() calculations.

Tech Stack

Next.js 14, React, Tailwind CSS.