Fix white/inverted flash when windows open

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>

docs/gotchas.md

@@ -49,6 +49,13 @@ touching the adjacent code.
   treatment is lost and the icon renders black on dark menu bars.
 - `ApplicationMenu` is macOS-only in our usage; accelerators on Windows are
   single-character only. ContextMenu exists on macOS+Windows, not Linux.
+- **Window-open flash prevention is three-layer** — remove any one and the
+  white/inverted blink returns: (1) windows are created `hidden: true` and
+  revealed by `showWhenReady` on the webview's first `dom-ready` (800 ms
+  fallback), (2) the bun preload bakes the persisted `data-theme`/tone/accent
+  attributes before first paint (`buildPreload`), (3) `ui/src/index.html`
+  carries an inline `<style>` with the html background per theme — its oklch
+  values MUST stay in sync with the `--background` tokens in styles.css.
 - Dev vs prod URL: `getMainViewUrl()` resolves the ng-serve dev server only
   when `DEV_SERVER_URL` is set AND the updater channel is `dev` AND the
   server answers a HEAD request; secondary windows must reuse the resolved