What changed

• observe macOS Low Power Mode changes in the battery view model
• render the menu bar battery fill in system yellow while Low Power Mode is active
• preserve the red critical-battery warning as the higher-priority state
• add coverage for icon states and live power-state notifications

Why

Juice should mirror the familiar macOS battery treatment so Low Power Mode is visible at a glance.

Verification

• make test (194 tests across 26 suites)
• git diff --check