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

components/figlet/FigletConverter.tsx

@@ -32,8 +32,8 @@ export function FigletConverter() {
   }, []);
 
   // Generate ASCII art
-  const generateAsciiArt = React.useCallback(
-    debounce(async (inputText: string, fontName: string) => {
+  const generateAsciiArt = React.useMemo(
+    () => debounce(async (inputText: string, fontName: string) => {
       if (!inputText) {
         setAsciiArt('');
         setIsLoading(false);