fix: cast hw_version to str for HA device registry compatibility

[DUC750]

Problem

NilanEntity.device_info passes hw_version as int, but the Home Assistant device registry requires str. This causes 82 warnings per HA startup on a CTS602 system:

ValueError: hw_version must be a string (got <class 'int'>)

Reported in: #224

Root Cause

In device.py, get_controller_hardware_version() returns int:

def get_controller_hardware_version(self) -> int:
    return self._device_hw_ver

In __init__.py, the value is passed directly to the device registry without casting:

"hw_version": self._device.get_device_hw_version,  # int → TypeError

Compare sw_version which already uses get_device_sw_version -> str and works correctly.

Fix

One-line change in custom_components/nilan/__init__.py:

"hw_version": str(self._device.get_device_hw_version),

Impact

• Fixes 82 warnings per HA restart (one per registered Nilan entity)
• Prevents breakage in a future HA release where this may be enforced as an error
• No functional change to integration behaviour

Tested on

• Home Assistant 2026.6.3
• Nilan CTS602 (82 entities)
• Warnings: 82 → 0 after restart