fix: null display name, settings scrollbar, redundant passkey offer

- Profile API returns empty string instead of "null" when Logto user
  has no display name set (String.valueOf(null) → "null" bug).
- SettingsPage: add overflowY auto + flex 1 so content scrolls within
  the AppShell layout (which uses overflow: hidden).
- Remove redundant passkey offer from onboarding page — passkey
  enrollment now happens during sign-in via the Experience API.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/tenant/SettingsPage.tsx

@@ -212,7 +212,7 @@ export function SettingsPage() {
   }
 
   return (
-    <div style={{ padding: 24, display: 'flex', flexDirection: 'column', gap: 20 }}>
+    <div style={{ padding: 24, display: 'flex', flexDirection: 'column', gap: 20, overflowY: 'auto', flex: 1 }}>
       <h1 style={{ margin: 0, fontSize: '1.25rem', fontWeight: 600 }}>Settings</h1>
       <PasskeyNudgeBanner />