Keep the system splash until the first real screen: no blank WebView gap, no second splash

Installed APK sequence was system splash -> white WebView -> web splash -> home. The
activity now holds the system splash (core-splashscreen) until the page calls
TadrisinoAndroid.appReady(), after its first non-boot screen; released after 10s or on a
main-frame load error so a dead site shows the WebView error, not an endless splash.
Older APKs have no appReady and are unaffected. Debug build compiles; not run on a device.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

android/app/src/main/AndroidManifest.xml

@@ -21,6 +21,7 @@
             android:name=".MainActivity"
             android:configChanges="orientation|screenSize|keyboardHidden"
             android:exported="true"
+            android:theme="@style/Theme.Tadrisino.Starting"
             android:screenOrientation="portrait">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />