FIX: missing SVG mask asset in wizard (PR #10856)

The dark-mode-friendly SVG mask for the wizard’s background image introduced in 8fcfb9586cb3b81e7862be48984d9d3c0b8f7d50 does not work with CDNs, because CORS restrictions apply to SVG masks.

It would be complicated to modify CDN access origin rules for this one specific assets, so instead, this PR moves the contents of the SVG file inside the stylesheet.