v1.2.0:2 — retry login/signup server action once on transport failure

iOS Safari reuses a keep-alive socket the server closed while the login
form sat idle during typing, so the first Sign In / Create account POST
dies instantly with NSURLErrorNetworkConnectionLost ("The network
connection was lost"). That rejects the server-action call, hitting the
client-side catch in LoginForm/SignupForm and showing "An unexpected
error occurred"; the second tap lands on a fresh connection and works.

Add lib/retryAction.ts: retryOnTransportError() retries the action once
only when the call throws. A returned { error } (bad password, rate
limit) is a real result and passes straight through. A lost-on-a-stale-
socket POST never reached the server, so retrying it once is safe.

proof-of-work/app/auth/login/LoginForm.tsx

@@ -3,6 +3,7 @@
 import { useState } from 'react';
 import { useRouter } from 'next/navigation';
 import { loginAction } from './actions';
+import { retryOnTransportError } from '@/lib/retryAction';
 
 export default function LoginForm() {
   const router = useRouter();
@@ -17,7 +18,9 @@ export default function LoginForm() {
     setLoading(true);
 
     try {
-      const result = await loginAction(email, password);
+      const result = await retryOnTransportError(() =>
+        loginAction(email, password)
+      );
 
       if (result.error) {
         setError(result.error);