fix: sync html lang attribute with i18n locale selection

- Set document.documentElement.lang on initial load and locale change
- Ensures screen readers and browser features respect the active language
- Removes duplicate aria-label on MessageSearch input

src/components/MessageSearch.tsx

@@ -62,8 +62,8 @@ export function MessageSearch({ open, onClose, onSearch, matchCount }: Props) {
         onChange={e => setQuery(e.target.value)}
         onKeyDown={handleKeyDown}
         placeholder={t('search.placeholder')}
-        className="bg-transparent text-sm text-pc-text placeholder:text-pc-text-muted outline-none w-48"
         aria-label={t('search.placeholder')}
+        className="bg-transparent text-sm text-pc-text placeholder:text-pc-text-muted outline-none w-48"
       />
       {query && (
         <span className="text-xs text-pc-text-muted whitespace-nowrap">