Separate focus-ring stroke and fill colors for transparent window support

[Alexb715]

Feature: Separate focus-ring border color from fill color

Currently, focus-ring uses a single active-color for both the visible border stroke and the filled rectangle rendered behind the window. This works great for opaque windows, but causes issues with transparent ones.

The problem:
When using a terminal like Alacritty with a low background opacity, the focus-ring's fill color bleeds through the window. For example, with active-color "#7e9cd8", the entire focused window gets a bright blue tint — not from the wallpaper, but from the focus-ring fill behind it.

Using an alpha value on active-color (#7e9cd840) reduces the bleed, but also dims the visible ring border itself, which isn't ideal.

Proposed solution:
Allow separate colors for the ring's stroke (visible border) and fill (area behind the window):

focus-ring {
    width 2
    active-color "#7e9cd8"
    active-fill-color "#00000000"  // fully transparent behind window
    inactive-color "#333333"
}

Or alternatively, simply don't fill the area behind the window at all — only draw the ring border.

Environment:

• Compositor: niri
• Terminal: Alacritty with opacity = 0.8
• Wallpaper: swaybg

Thanks for the great compositor!

For greater explanation i put transperancy