feat(valve): Make end-position current thresholds configurable

Introduces separate Modbus holding registers for configurable end-position
current thresholds for both opening and closing valve movements.

- Added REG_HOLDING_VALVE_END_CURRENT_THRESHOLD_OPEN_MA and
  REG_HOLDING_VALVE_END_CURRENT_THRESHOLD_CLOSE_MA to modbus_server.h.
- Modified valve.c to use these new thresholds and save/load them via settings.
- Added new setter functions to valve.h.
- Created new shell_valve library with commands to set/get these thresholds.
- Updated modbus_tool.py to include new menu options for setting thresholds.
- Updated docs/modbus-registers.de.md to document the new registers.

This enhances the flexibility and calibration of the valve control system.

software/include/lib/valve.h

@@ -99,6 +99,20 @@ void valve_set_max_open_time(uint16_t seconds);
  */
 void valve_set_max_close_time(uint16_t seconds);
 
+/**
+ * @brief Sets the current threshold for end-position detection during opening.
+ *
+ * @param current_ma The current threshold in milliamps.
+ */
+void valve_set_end_current_threshold_open(uint16_t current_ma);
+
+/**
+ * @brief Sets the current threshold for end-position detection during closing.
+ *
+ * @param current_ma The current threshold in milliamps.
+ */
+void valve_set_end_current_threshold_close(uint16_t current_ma);
+
 /**
  * @brief Gets the configured maximum opening time.
  *