Reduce risk of flash bang when opening the app in dark mode

Changelog: fix

Author: markharding

apps/polycentric/app/_layout.tsx

@@ -52,10 +52,10 @@ function RootStack() {
             ...); deep-linked profile/post views stay public everywhere. On
             web the tabs stay routable and get granular guards instead. */}
         <Stack.Protected guard={isWeb || accountGuard}>
-          <Stack.Screen name="(tabs)" />
+          <Stack.Screen name="(tabs)" options={{ animation: 'none' }} />
         </Stack.Protected>
 
-        <Stack.Screen name="(onboarding)" />
+        <Stack.Screen name="(onboarding)" options={{ animation: 'none' }} />
 
         {/* Account-only on every platform. Routes are auto-registered from
             the filesystem; the explicit declarations here exist to place

apps/polycentric/src/common/theme/provider.tsx

@@ -14,6 +14,7 @@ import {
   type PropsWithChildren,
 } from 'react';
 import { Appearance, useColorScheme } from 'react-native';
+import * as SystemUI from 'expo-system-ui';
 import { useSettings } from '@/src/common/settings';
 import { themes, type Theme, type ThemeKey } from './themes';
 
@@ -55,6 +56,13 @@ export function ThemeProvider({ children }: PropsWithChildren) {
 
   const theme = useMemo(() => themes[activeThemeName], [activeThemeName]);
 
+  // Keep the native window background in sync with the theme so the
+  // moments where no surface has painted (splash dismissal, stack
+  // transitions) don't flash the default white window.
+  useEffect(() => {
+    void SystemUI.setBackgroundColorAsync(theme.palette.neutral_0);
+  }, [theme]);
+
   const value = useMemo(
     () => ({ theme, activeThemeName, setActiveThemeName }),
     [theme, activeThemeName, setActiveThemeName],