fix(ui): restore useI18n import accidentally removed in cleanup

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Assisted-by: GLM-5.1 via Crush <crush@charm.land>

web/src/components/Studio.jsx

@@ -1,4 +1,5 @@
 import { useState, useRef, useEffect, useCallback } from 'react'
+import { useI18n } from '../i18n'
 import ReactMarkdown from 'react-markdown'
 import remarkGfm from 'remark-gfm'
 import remarkMath from 'remark-math'