feat: add gated Energy Saving menu

[ODevStudio]

Summary

• add the Energy Saving submenu behind HDS_ENABLE_ENERGY_MENU, with Back first
• keep the six existing toggles, add the guarded V8.1-only ACC Rail Off toggle, and show o/x rows with OFF/ON confirmations
• persist selections through the existing Preferences storage and remove Energy Status, Reset Stats, and status-only statistics
• expose energy-menu through the custom-build catalog without adding the dedicated firmware environment to the every-PR matrix

Verification

• python -m unittest discover -s tools -p 'test_*.py': 40 passed, 1 skipped
• pio test -e native: 28 passed
• focused energy, storage, soft-sleep, grinder flag, catalog, and custom-build contracts passed
• pio run -e esp32s3: passed
• pio run -e esp32s3-energy-menu: passed
• pio run -e esp32s3-energy-menu -t buildfs: passed
• WiFi-free esp32s3-custom build with only energy-menu: passed

CI and hardware

esp32s3-energy-menu is intentionally available for local/manual builds but is not compiled on every PR. The lightweight contract and native policy tests remain in PR CI.

Firmware and LittleFS were flashed successfully to a connected V8.1 scale. Interactive menu behavior and current draw still need physical qualification.