feat: add cryptographic utilities and types for secure data handling

webapp/src/components/HelpPage.tsx

@@ -0,0 +1,23 @@
+export default function HelpPage() {
+  return (
+    <div className="stack">
+      <section className="card">
+        <h3>Upstream Sync</h3>
+        <ul>
+          <li>Use fork + scheduled sync workflow.</li>
+          <li>Before merging, compare API routes and auth flow changes.</li>
+          <li>After merging, run migration tests in local dev before deploy.</li>
+        </ul>
+      </section>
+      <section className="card">
+        <h3>Common Errors</h3>
+        <ul>
+          <li>401 Unauthorized: token expired, log in again.</li>
+          <li>403 Account disabled: admin must unban your account.</li>
+          <li>403 Invite invalid: invite expired or revoked.</li>
+          <li>429 Too many requests: wait and retry.</li>
+        </ul>
+      </section>
+    </div>
+  );
+}