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.

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>