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/include/lib/valve.h

@@ -13,19 +13,9 @@
  * configuring the maximum opening and closing times.
  */
 
-/**
- * @brief Defines the GPIO pins used for the valve controller.
- */
-struct valve_gpios {
-  const struct gpio_dt_spec
-      in0; /**< Control input 0 for the VND7050AJ driver. */
-  const struct gpio_dt_spec
-      in1; /**< Control input 1 for the VND7050AJ driver. */
-  const struct gpio_dt_spec rst; /**< Reset pin for the VND7050AJ driver. */
-  const struct gpio_dt_spec sen; /**< Sense (current measurement) pin. */
-  const struct gpio_dt_spec s0;  /**< S0 select pin. */
-  const struct gpio_dt_spec s1;  /**< S1 select pin. */
-};
+#define VALVE_CHANNEL_OPEN 0
+#define VALVE_CHANNEL_CLOSE 1
+#define VALVE_ENDPOSITION_CHECK_INTERVAL K_MSEC(100)
 
 /**
  * @brief Represents the static state of the valve (open or closed).
@@ -49,8 +39,11 @@ enum valve_movement {
  * @brief Initializes the valve control system.
  *
  * Configures the GPIOs and loads saved settings for timeouts.
+ * This function must be called before any other valve functions.
+ *
+ * @return 0 on success, or a negative error code on failure.
  */
-void valve_init(void);
+int valve_init(void);
 
 /**
  * @brief Starts opening the valve.
@@ -120,4 +113,31 @@ uint16_t valve_get_max_open_time(void);
  */
 uint16_t valve_get_max_close_time(void);
 
+/**
+ * @brief Gets the current drawn by the valve motor during opening.
+ *
+ * @return The motor current in milliamps.
+ */
+int32_t valve_get_opening_current(void);
+
+/**
+ * @brief Gets the current drawn by the valve motor during closing.
+ *
+ * @return The motor current in milliamps.
+ */
+int32_t valve_get_closing_current(void);
+
+/**
+ * @brief Gets the temperature of the valve motor driver.
+ *
+ * @return The temperature in degrees Celsius.
+ */
+int32_t valve_get_vnd_temp(void);
+
+/**
+ * @brief Gets the voltage supplied to the valve motor driver.
+ *
+ * @return The voltage in millivolts.
+ */
+int32_t valve_get_vnd_voltage(void);
 #endif  // VALVE_H