feat(onboarding): anchor mnemonic to target letter

First word of each suggestion starts with the letter being learned
(e.g. "age WALK" for A). Validation requires at least one word to
start with the target letter. Added O/U/N/D/H words to fill gaps.

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

client/src/components/MnemonicBuilder.tsx

@@ -94,7 +94,8 @@ export function MnemonicBuilder({ token, onComplete }: Props) {
       </label>
       <p style={{ fontSize: '0.85rem', color: 'var(--color-brown)', marginBottom: '0.5rem' }}>
         {pattern.length} word{pattern.length !== 1 ? 's' : ''} —
-        {' '}≤3 letters for dots · (short), 4+ letters for dashes — (long).
+        {' '}≤3 letters for dots ·, 4+ letters for dashes —.
+        {' '}Start a word with "{letter}" to anchor it!
       </p>
       <input
         type="text"