Add real rack heatmap screenshot

[Hrabovszki1023]

Summary

• add a publication-ready PNG captured from the real five-module mixed US2000/US2000C rack in the upper charging range
• add a README section explaining relative per-module heatmap colors versus independent absolute-voltage markers
• avoid browser chrome, internal addresses and artificial fixture data

Screenshot content

The image shows live measurements for all 75 cells. It illustrates the intended diagnostic value: module 5 has a visible low cell and multiple higher cells while every absolute voltage remains below the configured balancing and warning thresholds.

Verification

• image visually inspected after copying into the repository
• PYTHONPATH=src python -m pytest -q — 207 passed
• python -m ruff check . — passed
• python -m mypy src tests — passed
• git diff --check — passed