Aero glass is transparent now, blurred without the per-frame cost

The white-ish fills are gone: every surface is a thin translucent glass with
the gloss and a bright rim, and the wallpaper reads through all of it.

The blur is a pre-blurred copy of the wallpaper (public/aero/
background-blur.webp, 3 kB) glued to the viewport with
background-attachment: fixed, layered under the tint and the gloss, instead
of a live backdrop-filter. Blurring the live scene measured 66ms per frame
at 2x CPU throttle (115 frames over 34ms) because the bubbles move behind
every surface; with the pre-blurred layer the same scene runs at 33ms and 8
slow frames — the same as the other themes, so the MacBook stays cool.

Sticky chrome, sheets and dropdowns keep a real backdrop-filter (they sit
over scrolling content, and their area is small).

Contrast: 0 below-minimum findings, 0 axe violations, all three themes.
Version 0.2.5, SW -> freezmo-v58.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "freezmo",
-  "version": "0.2.4",
+  "version": "0.2.5",
   "private": true,
   "license": "GPL-3.0-only",
   "scripts": {