feat(frontend): add countdown timer for active booking sessions

[mijinummi]

🎯 Summary

Implements a reusable countdown timer component for active booking sessions.

The component:

Displays remaining time in HH:MM:SS
Updates every second
Turns red when 5 minutes or less remain
Calls onExpire() exactly once when the timer reaches zero
Properly cleans up intervals on unmount
Includes comprehensive Jest fake timer tests

All implementation is contained within frontend/cntr/.
Closes #994

Implements a reusable step progress component for onboarding and booking workflows.

The component provides:

Completed, current, and upcoming step states
Visual progress tracking across multi-step flows
Accessible step indicators
Responsive horizontal layout
Connecting progress lines between steps
Comprehensive test coverage

All implementation is isolated to frontend/cntr/.

closes #995

feat: add reusable CopyButton component with clipboard fallback

Closes #996

✓ File exists at frontend/cntr/ReferralWidget/ReferralWidget.tsx
✓ Imports CopyButton
✓ Shows referral code
✓ WhatsApp URL uses encodeURIComponent
✓ Email URL uses subject and body query params
✓ Has unit tests
✓ No backend changes
✓ All code contained within frontend/cntr/

closes #997

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