refactor: apply React best practices and migrate to ESLint CLI

- Dynamic import for WordCloud component (code splitting)
- Fix waterfall in handleRefresh with Promise.all
- Memoize data prep functions in overview page (7 useMemo hooks)
- Replace useState+useEffect with useMemo in CountryDisplay/LanguageDisplay
- Memoize navigationCards and class getters in dashboard page
- Extract inline handlers with useCallback
- Fix missing index parameter in TopQuestionsChart map
- Migrate from next lint to ESLint CLI (Next.js 16 deprecation)

components/TopQuestionsChart.tsx

@@ -39,7 +39,7 @@ export default function TopQuestionsChart({
       </CardHeader>
       <CardContent>
         <div className="space-y-4">
-          {data.map((question) => {
+          {data.map((question, index) => {
             const percentage =
               maxCount > 0 ? (question.count / maxCount) * 100 : 0;