Split Nano firmware into modules

- config.h: all constants and debug defines
- command_parser.h/.cpp: CommandParser class, one method per command
- settings.h/.cpp: EEPROM flip time storage
- main.cpp: setup/loop and the periodic full redraw only
- Full redraw now checks frameEnded() after the serial buffer is drained
- Drop the SPI/Adafruit_I2CDevice include workaround (not needed)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/nano/settings.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <Arduino.h>
+
+// Settings that survive a power cycle (stored in EEPROM)
+
+// Stored flip time, or FLIP_TIME_DEFAULT when nothing valid is stored
+uint16_t loadFlipTime();
+
+void saveFlipTime(uint16_t t);