blur: revise general blur styling and refine it

cherry-pick of 1abb2210 from master, translated to hotfix-1.4.5
(omits ElevationShadow gating since shadow overhaul isn't on this
branch; gates existing MultiEffect-based shadows by BlurService.enabled
instead). Drops DoNotDisturbDetail.qml hunk since the file does not
exist on this branch.

quickshell/Modules/WorkspaceOverlays/NiriOverviewOverlay.qml

@@ -5,6 +5,7 @@ import Quickshell.Wayland
 import qs.Common
 import qs.Modals.DankLauncherV2
 import qs.Services
+import qs.Widgets
 
 Scope {
     id: niriOverviewScope
@@ -124,6 +125,17 @@ Scope {
                     item: overlayVisible && spotlightContainer.visible ? spotlightContainer : null
                 }
 
+                WindowBlur {
+                    targetWindow: overlayWindow
+                    readonly property real s: Math.min(1, spotlightContainer.scale)
+                    readonly property bool active: spotlightContainer.visible && spotlightContainer.opacity > 0
+                    blurX: spotlightContainer.x + spotlightContainer.width * (1 - s) * 0.5
+                    blurY: spotlightContainer.y + spotlightContainer.height * (1 - s) * 0.5
+                    blurWidth: active ? spotlightContainer.width * s : 0
+                    blurHeight: active ? spotlightContainer.height * s : 0
+                    blurRadius: Theme.cornerRadius
+                }
+
                 onShouldShowSpotlightChanged: {
                     if (shouldShowSpotlight) {
                         if (launcherContent?.controller) {