Add optional automatic remaining device power

[jefafafah]

Summary

This adds an optional virtual device that displays the portion of home
power consumption that is not accounted for by configured devices.

The value is calculated as:

home power - sum of configured device power

The result is clamped to zero.

Behavior

• The feature is opt-in via show_remaining_device: true.
• Existing configurations remain unchanged when it is disabled.
• No additional Home Assistant template sensor is required.
• The remaining device exists only inside the card.
• Invalid or unavailable values are handled safely (clamped to zero).
• Tapping the remaining device opens the more-info panel of the home entity.

Configuration

Setting	Default
show_remaining_device	false
remaining_device_name	Overige
remaining_device_icon	mdi:devices
remaining_device_color	#9e9e9e
remaining_device_threshold	5

Example:

show_remaining_device: true
remaining_device_name: Other
remaining_device_icon: mdi:help-circle
remaining_device_color: "#9e9e9e"
remaining_device_threshold: 10

Testing

• JavaScript syntax validation (node --check)
• Diff validation (no personal data, no debug code, no local configuration)