Stabilize mobile auth session handling

lib/auth-session.ts

@@ -0,0 +1,60 @@
+import type { createAuthClient } from "better-auth/react";
+
+import { authStoragePrefix } from "@/lib/accounts";
+import {
+  clearAuthStorage,
+  GUEST_AUTH_STORAGE_PREFIX,
+  prepareForAdditionalSignIn,
+} from "@/lib/auth-storage";
+
+type AuthClient = ReturnType<typeof createAuthClient>;
+
+type PerformAuthResetInput = {
+  authClient: AuthClient;
+  clearActiveAccount: () => Promise<void>;
+  activeAccountId?: string | null;
+  refetchSession?: () => Promise<unknown>;
+};
+
+/** Sign out, wipe local auth storage, and return to guest mode for a clean sign-in screen. */
+export async function performAuthReset({
+  authClient,
+  clearActiveAccount,
+  activeAccountId,
+  refetchSession,
+}: PerformAuthResetInput): Promise<void> {
+  const accountPrefix = activeAccountId ? authStoragePrefix(activeAccountId) : null;
+
+  try {
+    await authClient.signOut();
+  } catch {
+    // Continue clearing local state even when the server session is already gone.
+  }
+
+  if (accountPrefix) {
+    await clearAuthStorage(accountPrefix);
+  }
+  await clearAuthStorage(GUEST_AUTH_STORAGE_PREFIX);
+  await clearActiveAccount();
+  await refetchSession?.();
+}
+
+/**
+ * When the auth stack is shown, discard stale SecureStore sessions so sign-in starts clean.
+ * Expired account sessions switch back to guest storage; orphaned guest copies are cleared.
+ */
+export async function prepareAuthScreenSession(
+  authClient: AuthClient,
+  activeAccountId: string | null,
+  clearActiveAccount: () => Promise<void>,
+): Promise<void> {
+  const session = await authClient.getSession();
+  if (session.data?.user) return;
+
+  if (activeAccountId) {
+    await clearAuthStorage(authStoragePrefix(activeAccountId));
+    await clearActiveAccount();
+  }
+
+  await prepareForAdditionalSignIn();
+}