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/lib/guestAccountWarningSeen.ts

@@ -0,0 +1,47 @@
+import * as SecureStore from "expo-secure-store";
+import { Platform } from "react-native";
+
+const STORAGE_KEY = "guest_account_recovery_warning_seen";
+
+async function getStorage(): Promise<boolean> {
+    try {
+        let data: string | null = null;
+
+        if (Platform.OS === "web") {
+            if (typeof window !== "undefined" && typeof window.localStorage !== "undefined") {
+                data = window.localStorage.getItem(STORAGE_KEY);
+            }
+        } else {
+            data = await SecureStore.getItemAsync(STORAGE_KEY);
+        }
+
+        return data === "true";
+    } catch (error) {
+        console.error("Error reading guest warning state:", error);
+    }
+    return false;
+}
+
+async function setStorage(seen: boolean): Promise<void> {
+    try {
+        const data = seen ? "true" : "false";
+
+        if (Platform.OS === "web") {
+            if (typeof window !== "undefined" && typeof window.localStorage !== "undefined") {
+                window.localStorage.setItem(STORAGE_KEY, data);
+            }
+        } else {
+            await SecureStore.setItemAsync(STORAGE_KEY, data);
+        }
+    } catch (error) {
+        console.error("Error saving guest warning state:", error);
+    }
+}
+
+export async function hasSeenGuestAccountWarning(): Promise<boolean> {
+    return getStorage();
+}
+
+export async function markGuestAccountWarningSeen(): Promise<void> {
+    await setStorage(true);
+}