Feature: Offline credential storage with SD card module

Add support for an SPI SD card module to persist credentials and logs across reboots without relying on flash memory.\n\n**Acceptance criteria:**\n- [ ] Auto-detect SD card on boot\n- [ ] Append credentials to `credentials.csv` on SD\n- [ ] Append system logs to `log.txt` on SD\n- [ ] Dashboard indicator showing SD card status