perf(spa): drop backdrop blur on cards, fixed wallpaper layer, auto reduced effects on low-end (#24)

Reviewed-on: #24

spa/src/lib/effects.ts

@@ -0,0 +1,16 @@
+// Low-end devices can't keep up with live `backdrop-filter` while scrolling.
+// `deviceMemory` is Chromium-only (bucketed: 0.25 … 8 GB); Safari leaves it
+// undefined, so iPhones keep full effects. Flags <html> for aero-theme.css.
+
+type NavigatorWithMemory = Navigator & { deviceMemory?: number }
+
+function isLowEndDevice(): boolean {
+  const nav = navigator as NavigatorWithMemory
+  const lowMemory = nav.deviceMemory !== undefined && nav.deviceMemory <= 4
+  const fewCores = nav.hardwareConcurrency !== undefined && nav.hardwareConcurrency <= 2
+  return lowMemory || fewCores
+}
+
+export function applyEffectsLevel(): void {
+  if (isLowEndDevice()) document.documentElement.dataset.effects = "reduced"
+}