macOS configured no vibrancy surface at all, and an opaque fill sat between the desktop and the terminal, so Background Opacity and Window Blur both looked like no-ops.
Adds a platform-gated blur surface (`resolveMainWindowBlurSurface`, covering macOS vibrancy / Windows acrylic / Linux fallback) and a root class that drops the opaque fill so the opacity token reaches the compositor.
> [!WARNING]
> **This does not close#8797 — it is deliberately `Refs`, not `Fixes`.**
> I measured it on macOS with a dedicated isolated profile, identical app state, same wallpaper and window geometry, native `screencapture`, toggling only `windowBackgroundBlur` with a restart between runs:
>
> | Region | Blur OFF | Blur ON | Mean abs diff |
> | --- | --- | --- | --- |
> | Terminal pane | RGB(48.0, 53.0, 61.0) | RGB(50.0, 55.0, 63.1) | 2.0 / 255 |
> | Sidebar | RGB(46.3, 46.4, 46.4) | RGB(46.3, 46.4, 46.4) | 0 — pixel-identical |
>
> The renderer half provably works (`window-blur` class applied; `html`/`body`/`#root` compute to `rgba(0,0,0,0)`; `.xterm` at `rgba(40,44,52,0.55)`), but the native vibrancy never reaches the screen, so the setting is still not visibly effective. Most likely Electron keeps an opaque window backing without `transparent: true`, which #8482 removed on purpose.
>
> Kept because it is a correct, unit-tested prerequisite. Merge or hold at your discretion — one machine, one macOS version, external display.
Refs #8797
Co-authored-by: Orca <help@stably.ai>
