feat(settings): Implement persistent Modbus configuration

- Integrate the Zephyr Settings subsystem to persist Modbus parameters.
- Use NVS (Non-Volatile Storage) as the backend with a dedicated flash partition.
- Modbus baudrate and slave ID are now loaded at startup.
- Changes made via the shell are saved to flash and survive a reboot.
- Add a 'reset' command to the shell for easier testing.
- Fix all compiler and devicetree warnings for a clean build.

software/apps/slave_node/CMakeLists.txt

@@ -6,4 +6,4 @@ list(APPEND BOARD_ROOT ${CMAKE_CURRENT_SOURCE_DIR}/../..)
 find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
 project(slave_node)
 
-target_sources(app PRIVATE src/main.c src/shell_modbus.c)
+target_sources(app PRIVATE src/main.c src/shell_modbus.c src/shell_system.c)