fix: dark native chrome — no white flash on resize or first paint

Live-resize frame lag: the native window surface resizes before the
compositor has a frame at the new size; the gap showed Electron's
default WHITE window background — glaring on a dark app, at the edges
on shrink. The standard treatment, all three:

- BrowserWindow backgroundColor '#141414' fills the lag with the app bg
- show: false + ready-to-show — the window appears only after the page
  has rendered (kills the first-paint white window)
- color-scheme: dark — native widgets and scrollbars render dark

Owner resize-verified in the Electron dev shell.

app/globals.css

@@ -18,6 +18,7 @@
 /* Palette (owner's hexes): main #141414, raised #1C1C1C, edges #434343.
    Mono chrome = JetBrains Mono; prose = Source Sans 3; titles = CMU Serif. */
 :root {
+	color-scheme: dark;
 	--radius: 8px;
 	--background: #141414;
 	--foreground: #e6e6e6;