fix: resolve linting errors and improve ESLint configuration

- Downgrade ESLint to v9 to avoid circular structure errors in v10 config validation
- Downgrade eslint-config-next to v15 for stability
- Configure eslint.config.mjs with FlatCompat and appropriate ignores (.next, out)
- Escape entities in ColorBlindPage and SearchUnits to fix react/no-unescaped-entities
- Use useMemo for debounced function in FigletConverter to fix react-hooks/exhaustive-deps

eslint.config.mjs

@@ -10,6 +10,9 @@ const compat = new FlatCompat({
 });
 
 const eslintConfig = [
+  {
+    ignores: [".next/**", "out/**", "node_modules/**"],
+  },
   ...compat.extends("next/core-web-vitals"),
 ];