feat(slave): Implement VND7050AJ and core valve functionality; docs: Update German documentation and project plan

- Updated Doxygen comments in header files (valve.h, fwu.h, modbus_server.h) to be consistent and in English.
- Translated German register names in docs/modbus-registers.de.md to English.
- Updated docs/concept.de.md to reflect new details on current measurement and sensors.
- Updated docs/planning.de.md to reflect completed tasks in Phase 1.
- Implemented VND7050AJ and core functionality including current and voltage measurement and end-position detection.

software/apps/slave_node/src/main.c

@@ -1,4 +1,3 @@
-#include <app_version.h>
 #include <lib/fwu.h>
 #include <lib/modbus_server.h>
 #include <lib/valve.h>
@@ -9,9 +8,7 @@
 LOG_MODULE_REGISTER(main, LOG_LEVEL_INF);
 
 int main(void) {
-  int rc;
-  LOG_INF("Starting Irrigation System Slave Node version %s (Build version %s)",
-          APP_VERSION_STRING, STRINGIFY(APP_BUILD_VERSION));
+  LOG_INF("Starting Irrigation System Slave Node");
 
   if (settings_subsys_init() || settings_load()) {
     LOG_ERR("Settings initialization or loading failed");
@@ -20,9 +17,8 @@ int main(void) {
   valve_init();
   fwu_init();
 
-  rc = modbus_server_init();
-  if (rc < 0) {
-    LOG_ERR("Modbus RTU server initialization failed: %d", rc);
+  if (modbus_server_init()) {
+    LOG_ERR("Modbus RTU server initialization failed");
     return 0;
   }