on dark host pages the widget flashed an opaque white rectangle while loading.
the iframe element carries color-scheme from the theme param, but its document
had none until remark.tsx ran, and a mismatched color-scheme makes the embedded
canvas opaque instead of transparent. broken since #2023 added the element-side
color-scheme to fix a firefox dark-mode bug.

set the document's color-scheme from the theme param in an inline head script,
before first paint, using the same rule as create-iframe.ts. that closes the long
window but not the surface browsers paint before the document is parsed, which
webkit renders white and chromium hides behind paint holding. so also create the
iframe hidden and reveal it when the document posts inited, with a timeout
fallback so a failed bootstrap cannot leave the widget invisible.

the reveal lives in createIframe rather than embed.ts so the profile modal, the
other caller, gets it too. that modal focuses its iframe on open, and a hidden
element cannot take focus, so focus now fires from the reveal instead of a timer.

covered by a unit test for the reveal paths and the event.source guard, and by
e2e for the document's color-scheme and the iframe's visibility before inited,
after inited, and after the fallback.