fix: stop the cold start flashing white and the mail header jumping in

On a cold start the splash was followed by about a second of blank
white, then mail rows whose header sat under the status bar until it
popped in and pushed the list down.

- `AppTheme` had no `android:windowBackground`, so while JS loaded the
  window showed the platform default (near-white, or grey at night).
  It is now the app's own background, #ffffff by day and #09090b at
  night (`values-night`), matching `src/theme/tokens.ts`.
- `NavigationContainer` had no theme, so React Navigation painted its
  light default behind and between screens even in dark mode. It now
  uses the palette's background.
- The mail list took its top inset from the native `SafeAreaView`,
  which on Fabric only learns the insets a few frames after mounting.
  It now pads with `useSafeAreaInsets()`, which the navigator's
  safe-area provider fills from the initial window metrics on the first
  render; the value is the same, it just arrives in time.

App.tsx

@@ -2,7 +2,7 @@ import React from 'react';
 import { ActivityIndicator, AppState, Linking, StyleSheet, Text, View } from 'react-native';
 import { StatusBar } from 'expo-status-bar';
 import { useColorScheme } from 'react-native';
-import { NavigationContainer, createNavigationContainerRef } from '@react-navigation/native';
+import { DarkTheme, DefaultTheme, NavigationContainer, createNavigationContainerRef } from '@react-navigation/native';
 import { createBottomTabNavigator } from '@react-navigation/bottom-tabs';
 import { createNativeStackNavigator, type NativeStackScreenProps } from '@react-navigation/native-stack';
 import { Mail, Calendar, BookUser, HardDrive, Settings } from 'lucide-react-native';
@@ -267,6 +267,13 @@ export default function App() {
   const resolvedScheme: 'light' | 'dark' =
     themePref === 'system' ? (systemScheme === 'light' ? 'light' : 'dark') : themePref;
   const statusBarStyle: 'light' | 'dark' = resolvedScheme === 'light' ? 'dark' : 'light';
+  // React Navigation's default theme is light: without this its containers
+  // paint white behind and between screens, even in dark mode.
+  const background = useColors().background;
+  const navigationTheme = React.useMemo(() => {
+    const base = resolvedScheme === 'light' ? DefaultTheme : DarkTheme;
+    return { ...base, colors: { ...base.colors, background, card: background } };
+  }, [resolvedScheme, background]);
   // Persisted active account is the signal that the user was already signed
   // in on the previous launch. When present we render the main UI with the
   // cached mail list instead of the "Restoring session" spinner; the real
@@ -642,7 +649,7 @@ export default function App() {
   }
 
   return (
-    <NavigationContainer ref={navigationRef}>
+    <NavigationContainer ref={navigationRef} theme={navigationTheme}>
       <StatusBar style={statusBarStyle} />
       <Stack.Navigator screenOptions={{ headerShown: false }}>
         <Stack.Screen name="MainTabs" component={MainTabsNavigator} />

android/app/src/main/res/values-night/colors.xml

@@ -1 +1,4 @@
-<resources/>
+<resources>
+  <!-- Matches the dark palette background (src/theme/tokens.ts). -->
+  <color name="window_background">#09090b</color>
+</resources>

android/app/src/main/res/values/colors.xml

@@ -3,4 +3,6 @@
   <color name="iconBackground">#09090b</color>
   <color name="colorPrimary">#023c69</color>
   <color name="colorPrimaryDark">#09090b</color>
+  <!-- Behind the React root until the first frame; matches the light palette background (src/theme/tokens.ts). -->
+  <color name="window_background">#ffffff</color>
 </resources>

android/app/src/main/res/values/styles.xml

@@ -4,6 +4,7 @@
     <item name="android:editTextBackground">@drawable/rn_edit_text_material</item>
     <item name="colorPrimary">@color/colorPrimary</item>
     <item name="android:statusBarColor">#09090b</item>
+    <item name="android:windowBackground">@color/window_background</item>
   </style>
   <style name="Theme.App.SplashScreen" parent="AppTheme">
     <item name="android:windowBackground">@drawable/ic_launcher_background</item>

src/screens/EmailListScreen.tsx

@@ -1,6 +1,6 @@
 import React from 'react';
 import { View, Text, StyleSheet, FlatList, Pressable, TextInput, Image, ActivityIndicator, Modal, Platform, ScrollView, TouchableWithoutFeedback, Alert } from 'react-native';
-import { SafeAreaView } from 'react-native-safe-area-context';
+import { useSafeAreaInsets } from 'react-native-safe-area-context';
 import DateTimePicker, { type DateTimePickerEvent } from '@react-native-community/datetimepicker';
 import * as DocumentPicker from 'expo-document-picker';
 import {
@@ -295,6 +295,10 @@ interface EmailListScreenProps {
 export default function EmailListScreen({ onEmailPress, onComposePress }: EmailListScreenProps) {
   const c = useColors();
   const styles = React.useMemo(() => makeStyles(c), [c]);
+  // The navigator's safe-area provider knows the insets on the first render;
+  // the native SafeAreaView only learned them a few frames later on Fabric,
+  // so the header first drew under the status bar and then jumped down.
+  const insets = useSafeAreaInsets();
   const { t } = useLocaleStore();
   const [drawerOpen, setDrawerOpen] = React.useState(false);
   const [filterMenuOpen, setFilterMenuOpen] = React.useState(false);
@@ -917,7 +921,7 @@ export default function EmailListScreen({ onEmailPress, onComposePress }: EmailL
   }, [mailboxes, currentMailboxId, selectMailbox]);
 
   return (
-    <SafeAreaView style={styles.container} edges={['top']}>
+    <View style={[styles.container, { paddingTop: insets.top }]}>
       {/* Header */}
       {selectionMode ? (
         <View style={styles.header}>
@@ -1592,7 +1596,7 @@ export default function EmailListScreen({ onEmailPress, onComposePress }: EmailL
       />
 
       <UndoSnackbar />
-    </SafeAreaView>
+    </View>
   );
 }