Fix capacity tariff context and English units

[itsDNNS]

Summary

• use configured booked speeds or detected modem max rates for modulation capacity tariff context
• show Mbps in English capacity displays while keeping Mbit/s for German
• refresh service worker cache and add regressions for tariff fallback and units

Tests

• uv run --with-requirements requirements-test.txt pytest tests/test_modulation_api.py tests/test_modulation_static_contract.py tests/web/test_index.py tests/test_static_contracts.py tests/test_config.py
• uv run --with-requirements requirements-test.txt --with pytest-playwright pytest tests/e2e/test_modulation.py tests/e2e/test_pwa_gate.py
• uv run --with-requirements requirements-test.txt pytest tests --ignore=tests/e2e
• git diff --check