Add temperature offset sensor support

[mnaser]

Summary

• Add configurable temperature offset sensor in integration options
• When set, the offset value is subtracted from displayed temperatures and added to setpoints sent to the thermostat
• Useful for calibrating temperature readings when the thermostat is in a location that reads higher/lower than the actual room temperature

Use Case

If your thermostat reads 2°C higher than reality (e.g., it's near a heat source), you can create a template sensor with value 2 and configure it as the offset sensor. The integration will then:

• Show temperatures 2°C lower than what Daikin reports
• Send setpoints 2°C higher than what you configure in HA

Test plan

• Configure a template sensor with a numeric offset value
• Set the sensor in integration options
• Verify displayed current temperature is reduced by offset
• Verify target temperature setpoints are reduced by offset in display
• Verify setting a new target temperature adds the offset before sending to Daikin

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