feat: add next-themes package and update styles

- Added `next-themes` package for theme management.
- Updated various components to use `hsl(var(--foreground))` for color consistency.
- Changed background colors from `bg-white` to `bg-card` in multiple components for better theming support.

Made-with: Cursor

frontend/package.json

@@ -80,6 +80,7 @@
     "lucide-react": "^0.525.0",
     "mammoth": "^1.11.0",
     "next": "^15.4.8",
+    "next-themes": "^0.4.6",
     "qrcode": "^1.5.4",
     "react": "19.1.0",
     "react-day-picker": "^9.11.1",