Three layers: windows are created hidden and revealed on the webview's
first dom-ready (800ms fallback); the bun preload bakes the persisted
theme data-attributes so the first paint is already themed; and
index.html carries an inline background style covering the
pre-stylesheet paint (values mirrored from styles.css tokens).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>