- include/settings.h: Settings struct + SettingsManager class with
begin/load/save/reset and atomic save methods per value
- src/settings.cpp: input validation, clamping, default fallbacks;
password masked in printToSerial()
- src/main.cpp: call settings.begin() and printToSerial() on startup
- test_sketches/test_nvs.cpp: two-phase NVS persistence test;
write known values, reboot, verify all 7 values survive (PASS)
- platformio.ini: add test-nvs environment with settings.cpp in filter
- Implementation-Plan.md: mark tasks 1.4 and 2.1/2.2 complete
