Fix availableEnergyCapacity display unit in periodic_requests example

[eman]

Problem

The periodic_requests.py example was incorrectly displaying availableEnergyCapacity as a percentage (e.g., "Available Energy: 709.0%"), which doesn't make sense since 709% is not a valid percentage value.

Root Cause

According to the documentation in docs/DEVICE_STATUS_FIELDS.rst, the availableEnergyCapacity field represents:

• Units: Watt-hours (Wh)
• Description: Available energy capacity - remaining hot water energy available in Watt-hours

This is an energy measurement, not a percentage.

Solution

Changed the display format in the on_device_status callback from:

print(f"  Available Energy: {status.availableEnergyCapacity:.1f}%")

To:

print(f"  Available Energy: {status.availableEnergyCapacity:.0f} Wh")

Results

Before: Available Energy: 709.0% (confusing)
After: Available Energy: 709 Wh (correct)

Verification

• Reviewed all other examples to ensure no similar issues exist
• Confirmed that other percentage displays (e.g., dhwChargePer) are correctly used
• Code passes linting and formatting checks

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature
• Breaking change
• Documentation update (example fix)

This is a minor correction that improves example accuracy and user understanding.