feat(mobile): auth callback screen consumes magic-link tokens

apps/mobile/app/auth/callback.tsx

@@ -0,0 +1,77 @@
+import { useLocalSearchParams, useRouter } from 'expo-router';
+import { useEffect, useState } from 'react';
+import { ActivityIndicator, StyleSheet, Text, View } from 'react-native';
+
+import { supabase } from '../../lib/supabase';
+import { colors } from '../../theme/colors';
+
+// Supabase magic-link emails redirect to netralax://auth/callback with
+// the tokens in either the URL fragment (#access_token=...&refresh_token=...)
+// or the query string depending on the provider. Expo Router parses the
+// query string into useLocalSearchParams. The hash portion would require
+// expo-linking; we accept both shapes for safety.
+export default function AuthCallback() {
+  const params = useLocalSearchParams<{
+    access_token?: string;
+    refresh_token?: string;
+    error?: string;
+    error_description?: string;
+  }>();
+  const router = useRouter();
+  const [status, setStatus] = useState<'working' | 'error'>('working');
+  const [message, setMessage] = useState<string>('');
+
+  useEffect(() => {
+    void (async () => {
+      if (params.error) {
+        setStatus('error');
+        setMessage(params.error_description ?? params.error);
+        return;
+      }
+      if (!params.access_token || !params.refresh_token) {
+        setStatus('error');
+        setMessage('Magic-Link-URL enthielt keine Tokens.');
+        return;
+      }
+      const { error } = await supabase.auth.setSession({
+        access_token: params.access_token,
+        refresh_token: params.refresh_token,
+      });
+      if (error) {
+        setStatus('error');
+        setMessage(error.message);
+        return;
+      }
+      router.replace('/(app)/chats');
+    })();
+  }, [params, router]);
+
+  return (
+    <View style={styles.container}>
+      {status === 'working' ? (
+        <>
+          <ActivityIndicator color={colors.accent} size="large" />
+          <Text style={styles.text}>Du wirst angemeldet…</Text>
+        </>
+      ) : (
+        <>
+          <Text style={styles.errorTitle}>Anmeldung fehlgeschlagen</Text>
+          <Text style={styles.text}>{message}</Text>
+        </>
+      )}
+    </View>
+  );
+}
+
+const styles = StyleSheet.create({
+  container: {
+    flex: 1,
+    alignItems: 'center',
+    justifyContent: 'center',
+    backgroundColor: colors.bg,
+    padding: 24,
+    gap: 16,
+  },
+  text: { color: colors.textMuted, fontSize: 14, textAlign: 'center' },
+  errorTitle: { color: colors.danger, fontSize: 18, fontWeight: '600' },
+});