Add APY utilization stress panel

[Sikkra]

Summary

• Adds a utilization stress-test panel below the leverage preview with 80%, 90%, 95%, 99%, and custom utilization scenarios.
• Recomputes stressed supply/borrow rates through projectRates and shows net APY, 30-day HF, and liquidation timing for the selected leverage.
• Flags 95%+ scenarios as the r_three kink zone and updates demo reserves so demo mode exercises the same rate model.

Verification

• npm run build
• git diff --check
• Headless Edge/CDP check on http://127.0.0.1:5197/: demo mode rendered 5 stress rows; 95%, 99%, and 99% custom rows were marked as r_three kink; no console warnings/errors or runtime exceptions were reported.

Closes #34