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)

eslint.config.js

@@ -1,3 +1,5 @@
+import nextTypescript from "eslint-config-next/typescript";
+import nextCoreWebVitals from "eslint-config-next/core-web-vitals";
 import path from "node:path";
 import { fileURLToPath } from "node:url";
 import { FlatCompat } from "@eslint/eslintrc";
@@ -10,11 +12,10 @@ const compat = new FlatCompat({
 });
 
 const eslintConfig = [
+  ...nextTypescript,
   js.configs.recommended,
-  ...compat.extends(
-    "next/core-web-vitals",
-    "plugin:@typescript-eslint/recommended"
-  ),
+  ...nextCoreWebVitals,
+  ...compat.extends("plugin:@typescript-eslint/recommended"),
   {
     ignores: [
       "node_modules/",
@@ -34,7 +35,7 @@ const eslintConfig = [
       "prefer-const": "error",
       "no-unused-vars": "warn",
     },
-  },
+  }
 ];
 
 export default eslintConfig;