debug: add crash boundary and global error handler to surface release crashes

Catches React render errors (ErrorBoundary) and fatal JS errors
(ErrorUtils.setGlobalHandler) and displays the stacktrace on-screen
instead of crashing to desktop. Temporary — remove after crash identified.

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

app/_layout.tsx

@@ -1,13 +1,55 @@
-import { useEffect } from "react";
+import { useEffect, useState, Component } from "react";
+import type { ReactNode } from "react";
 import { Stack, router } from "expo-router";
 import { StatusBar } from "expo-status-bar";
 import { GestureHandlerRootView } from "react-native-gesture-handler";
+import { View, Text, ScrollView } from "react-native";
 import "../global.css";
 import { Colors } from "@/constants/theme";
 import { useSettings } from "@/hooks/useSettings";
 
+// Catch JS errors in release mode and show them on-screen instead of
+// silently crashing to desktop. Remove once the crash is identified.
+class CrashBoundary extends Component<
+  { children: ReactNode },
+  { error: Error | null }
+> {
+  state = { error: null };
+  static getDerivedStateFromError(e: Error) {
+    return { error: e };
+  }
+  render() {
+    const { error } = this.state;
+    if (error) {
+      return (
+        <View style={{ flex: 1, backgroundColor: "#0f1117", padding: 20, paddingTop: 60 }}>
+          <Text style={{ color: "#ef4444", fontSize: 16, fontWeight: "700", marginBottom: 12 }}>
+            CRASH — copie ce texte
+          </Text>
+          <ScrollView>
+            <Text style={{ color: "#fff", fontSize: 11, fontFamily: "monospace" }}>
+              {String(error)}{"\n\n"}{(error as any).stack}
+            </Text>
+          </ScrollView>
+        </View>
+      );
+    }
+    return this.props.children;
+  }
+}
+
 export default function RootLayout() {
   const { settings, ready } = useSettings();
+  const [jsError, setJsError] = useState<string | null>(null);
+
+  useEffect(() => {
+    const prev = (ErrorUtils as any).getGlobalHandler();
+    (ErrorUtils as any).setGlobalHandler((e: Error, isFatal: boolean) => {
+      if (isFatal) setJsError(`${String(e)}\n\n${(e as any).stack ?? ""}`);
+      prev?.(e, isFatal);
+    });
+    return () => (ErrorUtils as any).setGlobalHandler(prev);
+  }, []);
 
   useEffect(() => {
     if (!ready) return;
@@ -16,7 +58,23 @@ export default function RootLayout() {
     }
   }, [ready, settings.onboardingDone]);
 
+  if (jsError) {
     return (
+      <View style={{ flex: 1, backgroundColor: "#0f1117", padding: 20, paddingTop: 60 }}>
+        <Text style={{ color: "#ef4444", fontSize: 16, fontWeight: "700", marginBottom: 12 }}>
+          FATAL JS ERROR — copie ce texte
+        </Text>
+        <ScrollView>
+          <Text style={{ color: "#fff", fontSize: 11, fontFamily: "monospace" }}>
+            {jsError}
+          </Text>
+        </ScrollView>
+      </View>
+    );
+  }
+
+  return (
+    <CrashBoundary>
       <GestureHandlerRootView style={{ flex: 1, backgroundColor: Colors.bg }}>
         <StatusBar style="light" />
         <Stack
@@ -35,5 +93,6 @@ export default function RootLayout() {
           <Stack.Screen name="onboarding" options={{ headerShown: false }} />
         </Stack>
       </GestureHandlerRootView>
+    </CrashBoundary>
   );
 }