Enhancement: migrate OverlayProcess background to kagerou

[hibiyasleep]

Problem 1

Sentry log (not helpful): https://s.hibiya.moe/share/issue/322e36/

Reason: OverlayWindow's background feature try to access document.body.style, even document isn't fully loaded.

This exception was reported 6.8k times from 3.5k users, which all from OverlayProcess users.

cc @ZCube

[hibiyasleep]

Enhancement

Detect document.body.style.backgroundImage, then migrate it to config.style.table-bg-image. (+ notification)