Add a no-animations mode, on by default in VMs (#13550)

A machine without a GPU, like most VMs, renders through llvmpipe on the
CPU, where every animated frame and every translucent window costs. In a
VM, opening and closing a terminal took ~5s of CPU; a panel ~3.4s.

omarchy toggle animations (also under Toggle > Animations) places a Hyprland
flag that turns off animations, blur and shadows and makes windows opaque.
The shell follows Hyprland's animations:enabled, rereading it on every
config reload: its one-shot animations run for Style.duration(ms), which
is then 0, and its spinners, pulses and title marquee hold still. A new
install in a VM starts with the flag in place.

Measured in the ISO test VM (llvmpipe), CPU per interaction:
terminal open+close 5000ms -> 481ms, workspace switch 1670ms -> 409ms,
audio panel 3446ms -> 796ms, volume OSD 2324ms -> 584ms, menu 2028ms ->
1241ms.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

shell/plugins/background/Background.qml

@@ -258,7 +258,7 @@ Item {
     property: "revealProgress"
     from: 0
     to: 1
-    duration: 420
+    duration: Style.duration(420)
     easing.type: Easing.InOutCubic
     onFinished: {
       if (root.incomingBackground) {