feat(onboarding): curated meaningful mnemonic phrases

Replace random word banks with hand-crafted mnemonic suggestions
for each letter. All phrases are meaningful (e.g. "he has his hat"
for H, "sun set sky" for S, "over only once" for O) and every word
starts with the target letter. Placeholder shows first suggestion.

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

client/src/components/MnemonicBuilder.tsx

@@ -101,7 +101,7 @@ export function MnemonicBuilder({ token, onComplete }: Props) {
         type="text"
         value={phrase}
         onChange={e => { setPhrase(e.target.value); setError(null) }}
-        placeholder={`e.g. "${examplePhrase(pattern)}"`}
+        placeholder={suggestions.length > 0 ? `e.g. "${suggestions[0]}"` : ''}
         disabled={saving}
         style={{ marginBottom: '0.5rem' }}
       />
@@ -197,8 +197,3 @@ export function MnemonicBuilder({ token, onComplete }: Props) {
     </div>
   )
 }
-
-// Generate a simple example phrase for the placeholder
-function examplePhrase(pattern: string): string {
-  return pattern.split('').map(c => c === '.' ? 'run' : 'WALK').join(' ')
-}