Stabilize mobile auth session handling

lib/auth-cookie.ts

@@ -0,0 +1,55 @@
+import { getCookie as serializeStoredCookies } from "@better-auth/expo/client";
+import * as SecureStore from "expo-secure-store";
+import type { createAuthClient } from "better-auth/react";
+
+import { normalizeSecureStoreKey } from "@/lib/secure-store-keys";
+
+type AuthClient = ReturnType<typeof createAuthClient>;
+
+const CHUNK_MARKER = "\u0001ba-chunks:";
+
+function readSecureStoreValueSync(key: string): string | null {
+  const value = SecureStore.getItem(key);
+  if (value == null) return null;
+  if (!value.startsWith(CHUNK_MARKER)) return value;
+
+  const count = Number(value.slice(CHUNK_MARKER.length));
+  if (!Number.isInteger(count) || count < 1) return null;
+
+  let assembled = "";
+  for (let index = 0; index < count; index += 1) {
+    const chunk = SecureStore.getItem(`${key}.${index}`);
+    if (chunk == null) return null;
+    assembled += chunk;
+  }
+  return assembled;
+}
+
+/** Read session cookie string for tRPC requests (Expo client plugin + SecureStore fallback). */
+export function getAuthCookie(
+  authClient: AuthClient,
+  storagePrefix: string,
+): string | null {
+  const fromClient = (
+    authClient as AuthClient & { getCookie?: () => string }
+  ).getCookie?.();
+  if (fromClient?.trim()) return fromClient.trim();
+
+  const raw = readSecureStoreValueSync(
+    normalizeSecureStoreKey(`${storagePrefix}_cookie`),
+  );
+  if (!raw || raw === "{}") return null;
+
+  const cookie = serializeStoredCookies(raw);
+  return cookie.trim() || null;
+}
+
+export function getAuthCookieHeaders(
+  authClient: AuthClient,
+  storagePrefix: string,
+): Record<string, string> {
+  const cookie = getAuthCookie(authClient, storagePrefix);
+  return cookie
+    ? { cookie, Cookie: cookie, "x-beenvoice-auth-cookie": cookie }
+    : {};
+}