This fixes applying display brightness from the current profile
configuration at startup. This had failed on Wayland since 6.2
when KWin brightness integration was added. Here's why.

In a Wayland session, the `ScreenBrightnessControl` action will not
initially announce itself as supported because KWin outputs are only
known after a brief asynchronous delay. Instead, it gets enabled
as a reaction to the first `displayAdded`/`displayIdsChanged` signal.

The `refreshActions()` slot that handles this would create and
register the action object, but the action's `loadAction()` and
`onProfileLoaded()` methods were not called. Because the profile
was already loaded earlier, these methods would not get called
until the next profile change (or wake-up from sleep).

We now take care to call these action methods, but only when the
profile is active. They won't be called during Core initialization,
and if the login session is currently inactive.

BUG: 497362