feat: transition for opacity changes, including "flash" effect

[perstarkse]

Hi,

Resubmitting as a superceeding PR since in the previous PR #2804 the user sashaduke had a feature request. When considering it, I felt integrating it made a lot of sense, since they share a lot of overlap.

This PR introduces two complementary features.
1, Smooth opacity transitions when changing focus between two windows of different opacity. Requested in #1801.
2, Focus flash, as discussed in #2804 and #1380, which provides visual feedback via a short opacity change when a window is focused. Making it easier to see which window is focused. Especially when not using borders or focus rings.

niri-demo-vp9-crf26.webm

The new settings goes into the layout block

layout {
    opacity-transition {
        on                    // Enable smooth transitions
        duration-ms 350       // Animation duration for both transitions and flash
        flash {
            on                // Enable focus flash effect
            flash-opacity 0.8 // Target opacity during flash (0.0 - 1.0)
            disable-on-solo   // Optional: disable flash for solo windows
        }
    }
}

[sashaduke]

Great work, many have requested this and it builds a solid foundation for future related work/features. Big +1 from me