Depends on #3155

This PR ensures that the External Power (ext_power) state is correctly set with the RGB Underglow state. It addresses two distinct issues:

• Pre-existing Bug: The initial RGB state was not being correctly applied to the hardware after a settings_reset or a bootloader flash when CONFIG_ZMK_RGB_UNDERGLOW_ON_START=n is set as ext_power defaults to on.
• Regression (from Fix ext_power state persistence #3155): A side effect of the linked PR caused the ext_power state to fail to initialize correctly after a simple power cycle (On/Off). Again, as ext_power defaults to on.

I still need to verify these changes across different scenarios to ensure no other side effects were introduced so I'm opening this as a draft for now.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.