fix(theme): kill flash of white on dark-themed load

The app painted white until <ThemeProvider> mounted and set data-theme,
then snapped to the dark theme. Two fixes:

- Inline head script (rsbuild html.tags) sets data-theme before first
  paint, mirroring modern-sk's exact logic (localStorage 'modern-sk-theme'
  || 'dark') so there's no second flip when the provider mounts. Inline =
  zero round-trips.
- body now paints var(--color-bg) so the themed background shows before
  React mounts #root and layers the felt grain on top.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/styles/global.css

@@ -28,6 +28,11 @@ body {
   margin: 0;
   font-family: var(--font-sans);
   color: var(--fg-1);
+  /* Paint the themed background immediately. The inline theme script in
+     index.html (see rsbuild.config.ts) sets [data-theme] before first paint, so
+     --color-bg resolves to the right value here before React mounts #root and
+     layers the .modern-sk-felt grain on top — no flash of white. */
+  background: var(--color-bg);
   -webkit-font-smoothing: antialiased;
 }