feat: config module — NVS read/write via Preferences

Add config.h/config.cpp for DeviceConfig NVS persistence using Arduino
Preferences library. Add minimal main.cpp stub. Fix partition table
overlap (nvs 0x6000→0x5000, otadata 0xf000→0xe000) so firmware builds.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

firmware/src/main.cpp

@@ -0,0 +1,3 @@
+#include <Arduino.h>
+void setup() {}
+void loop() {}