[FE-34] Build QR Code Check-In Display component with expiry countdown

[devwums]

Problem

The check-in flow references QR codes but no component exists to display the generated QR code for a session. The feature is implemented on the backend but invisible on the frontend.

Proposed Solution

Create frontend/cntr/QRCheckIn/QRCheckInDisplay.tsx. Props: qrDataUrl: string, workspaceName: string, expiresAt: string. Renders the workspace name, the QR code as <img src={qrDataUrl}>, and a countdown to expiry using the CountdownTimer component from frontend/cntr/CountdownTimer/. When the timer reaches zero, replaces the QR code with "QR code expired — request a new one." All implementation must live inside frontend/cntr/.

Acceptance Criteria

• File at frontend/cntr/QRCheckIn/QRCheckInDisplay.tsx
• Renders <img src={qrDataUrl} alt="Check-in QR Code">
• Workspace name displayed above the QR code
• Uses CountdownTimer component from ../CountdownTimer/CountdownTimer
• Shows expiry message when timer reaches zero (via onExpire callback)
• Test in frontend/cntr/QRCheckIn/QRCheckInDisplay.test.tsx