Add Serbian translations and dark mode support

- Translated all UI text to Serbian
- Added dark mode toggle with system preference support
- Fixed text contrast on colored table rows
- Added dark mode styles throughout the app

tailwind.config.js

@@ -1,5 +1,6 @@
 /** @type {import('tailwindcss').Config} */
 export default {
+  darkMode: 'class',
   content: [
     "./index.html",
     "./src/**/*.{js,ts,jsx,tsx}",