improvement(shell-ui): paint the theme background before the app loads (ARTESCA-18437)

[Cuervino]

Every page load shows a white flash before the shell renders, even in dark mode. The HTML template sets no background, so the browser paints its default white until React mounts and the theme applies.

This adds a small inline script to index-template.html that reads the theme already stored by ShellThemeSelectorProvider (localStorage.theme, dark by default) and sets the matching background on <html> before anything else loads.

The two colors are the backgroundLevel1 of the themes ARTESCA configures (darkRebrand and artescaLight). They are hard-coded because the theme is not available yet at that point.

Fixes ARTESCA-18437.

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[Cuervino]

/changie Enhanced Remove the white flash on page load in dark mode

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[claude]

These inline styles on <html> are never updated afterwards. When the user switches theme from the navbar (setThemeMode in ShellThemeSelectorProvider.tsx), <html> keeps the old background and color-scheme until reload. In light mode that leaves dark scrollbars, dark native form controls and a dark overscroll area. Fix: update document.documentElement.style.backgroundColor/colorScheme in changeThemeMode (or in an effect on themeMode, using selectedTheme.backgroundLevel1), or remove the inline styles once the app has mounted.

— Claude Code

[claude]

• <html> background and color-scheme set in index-template.html are not updated when the theme is switched at runtime, so they stay out of sync until reload
    - Update them in ShellThemeSelectorProvider.changeThemeMode (or an effect on themeMode) using selectedTheme.backgroundLevel1
    - Or clear the inline styles once the app has mounted
  
  Review by Claude Code

[JeanMarcMilletScality]

One gap: the script sets background-color and color-scheme on once, at load time, and nothing updates them afterwards. After switching from dark to light
without reloading, keeps the dark background and color-scheme: dark until the next reload.

I suggest letting the provider take over once it mounts, so the values come from the real theme after the first paint:

// ShellThemeSelectorProvider.tsx
useEffect(() => {
const root = document.documentElement;
root.style.backgroundColor = selectedTheme.backgroundLevel1;
root.style.colorScheme = themeMode;
}, [selectedTheme, themeMode]);

Optional: the themes come from config.json, so the two hard-coded colors can drift. The provider could store backgroundLevel1 in
localStorage for the script to read, with the current colors as a fallback for the first visit.