feat: add theme toggle functionality and update localization files

- Implemented a theme toggle feature allowing users to switch between light and dark modes.
- Added a new ThemeToggle component for the theme switching button.
- Updated theme CSS to support dark mode variables and system preference detection.
- Enhanced localization files to include translations for the "toggleTheme" key across multiple languages.

src/locales/fi.json

@@ -26,7 +26,8 @@
     "logout": "Kirjaudu ulos",
     "login": "Kirjaudu sisään",
     "register": "Rekisteröidy",
-    "language": "Kieli"
+    "language": "Kieli",
+    "toggleTheme": "Vaihda teemaa"
   },
   "convert": {
     "title": "Muunna",