revert: remove all splash screen overrides, revert app.json to v0.1.5 state

SplashScreen.hideAsync() and the splash section in app.json were both
introduced after v0.1.5 and are common to every crashing build.
Reverting to the exact v0.1.5 baseline to isolate the crash source.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/_layout.tsx

@@ -1,5 +1,5 @@
 import { useEffect } from "react";
-import { Stack, router, SplashScreen } from "expo-router";
+import { Stack, router } from "expo-router";
 import { StatusBar } from "expo-status-bar";
 import { GestureHandlerRootView } from "react-native-gesture-handler";
 import "../global.css";
@@ -9,11 +9,6 @@ import { useSettings } from "@/hooks/useSettings";
 export default function RootLayout() {
   const { settings, ready } = useSettings();
 
-  useEffect(() => {
-    if (!ready) return;
-    SplashScreen.hideAsync();
-  }, [ready]);
-
   useEffect(() => {
     if (!ready) return;
     if (!settings.onboardingDone) {