feat: onboarding mnemonic builder with step-through UI (Task 10)

- MnemonicBuilder: 12 NOVICE_LETTERS, real-time validatePhrase, save/skip
- OnboardingPage: wraps builder, passes token and onComplete
- App.tsx: post-auth fetch /api/mnemonics, route to onboarding if <12 saved
- Fix: validatePhrase called with letter (not Morse pattern) as second arg

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

client/src/pages/OnboardingPage.tsx

@@ -0,0 +1,20 @@
+import { MnemonicBuilder } from '../components/MnemonicBuilder'
+
+type Props = {
+  token: string
+  onComplete: () => void
+}
+
+export function OnboardingPage({ token, onComplete }: Props) {
+  return (
+    <div>
+      <h1 style={{ fontFamily: 'var(--font-header)', marginBottom: '0.5rem' }}>
+        Build Your Mnemonics
+      </h1>
+      <p style={{ color: 'var(--color-brown)', marginBottom: '2rem' }}>
+        Create a memorable phrase for each Morse letter. Short words for dots, long words for dashes.
+      </p>
+      <MnemonicBuilder token={token} onComplete={onComplete} />
+    </div>
+  )
+}