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/modbus_server.h

@@ -88,6 +88,10 @@ enum {
 	 * @brief Safety timeout in seconds for the closing process.
 	 */
 	REG_HOLDING_MAX_CLOSING_TIME_S = 0x0002,
+	/**
+	 * @brief Minimum current threshold in mA for end-position detection.
+	 */
+	REG_HOLDING_VALVE_END_CURRENT_THRESHOLD_MA = 0x0003,
 	/**
 	 * @brief Bitmask for reading and writing digital outputs. Bit 0: Output 1,
 	 * Bit 1: Output 2. 1=ON, 0=OFF.