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

package.json

@@ -6,7 +6,7 @@
     "dev": "next dev --turbopack",
     "build": "next build",
     "start": "next start",
-    "lint": "next lint"
+    "lint": "eslint ."
   },
   "dependencies": {
     "@tanstack/react-query": "^5.90.21",
@@ -28,13 +28,14 @@
     "zustand": "^5.0.11"
   },
   "devDependencies": {
+    "@eslint/eslintrc": "^3.3.3",
     "@tailwindcss/postcss": "^4.2.0",
     "@types/figlet": "^1.7.0",
     "@types/node": "^25.3.0",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
-    "eslint": "^10.0.1",
-    "eslint-config-next": "^16.1.6",
+    "eslint": "^9.21.0",
+    "eslint-config-next": "^15.1.7",
     "postcss": "^8.5.6",
     "tailwindcss": "^4.2.0",
     "typescript": "^5.9.3"