Add guest auth and save-account flow

Introduce anonymous sign-in with in-app saving, guest recovery warnings, paywall prompts, and admin guest labeling.

apps/mobile/src/features/profile/screens/AccountScreen.tsx

@@ -1,5 +1,6 @@
-import React from 'react';
-import { View, ScrollView, StyleSheet, Platform } from 'react-native';
+import React from 'react';
+import { View, ScrollView, StyleSheet, Platform } from 'react-native';
+import { useRouter } from 'expo-router';
 
 import { GradientBackground } from '../../../components/ui';
 import { colors, spacing } from '../../../theme';
@@ -10,9 +11,10 @@ import { ScreenHeader, SettingsSection, MenuItem, DangerZone } from '../componen
 /**
  * Account settings sub-screen (sign out, delete account).
  */
-export function AccountScreen() {
-    const { couple } = useAuthStore();
-    const { handleSignOut, handleDeleteAccount } = useCoupleManagement();
+export function AccountScreen() {
+    const router = useRouter();
+    const { couple, isAnonymous } = useAuthStore();
+    const { handleSignOut, handleDeleteAccount } = useCoupleManagement();
 
     return (
         <GradientBackground>
@@ -22,15 +24,25 @@ export function AccountScreen() {
                 contentContainerStyle={styles.content}
                 showsVerticalScrollIndicator={false}
             >
-                <SettingsSection title="Account" delay={0}>
-                    <MenuItem
-                        icon="log-out-outline"
-                        label="Sign Out"
-                        onPress={handleSignOut}
-                        variant="danger"
-                        showChevron={false}
-                    />
-                </SettingsSection>
+                <SettingsSection title="Account" delay={0}>
+                    {isAnonymous ? (
+                        <MenuItem
+                            icon="shield-checkmark-outline"
+                            label="Save Account"
+                            description="Protect access across devices"
+                            onPress={() => router.push("/(app)/settings/save-account" as any)}
+                            showChevron={false}
+                        />
+                    ) : (
+                        <MenuItem
+                            icon="log-out-outline"
+                            label="Sign Out"
+                            onPress={handleSignOut}
+                            variant="danger"
+                            showChevron={false}
+                        />
+                    )}
+                </SettingsSection>
 
                 <View style={styles.spacer} />