feat: password unlock screen on refresh instead of full logout

On page reload, tokens persist but crypto keys are lost (by design).
Now shows a password-only unlock screen instead of the full login form,
so users don't need to re-enter their email after a refresh.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/RegisterPage.tsx

@@ -52,7 +52,7 @@ export default function RegisterPage() {
       });
 
       const { accessToken, refreshToken, userId } = res.data;
-      setAuth({ accessToken, refreshToken, userId });
+      setAuth({ accessToken, refreshToken, userId, email });
       setMasterKey(masterKey);
       setKeyPair(publicKeyB64, privateKey);
       setUsername(username);