Refactor login functionality and enhance MobileCallPage positioning

- Update LoginPage to initialize username state as an empty string for improved user experience.
- Introduce floatingVideoRef in MobileCallVisualWorkspace to dynamically manage video panel positioning.
- Adjust position calculations for the floating video panel to account for its dimensions, enhancing drag-and-drop functionality and ensuring proper placement within the viewport.

frontend/src/app/login/page.tsx

@@ -18,7 +18,7 @@ import { Input } from "@/components/ui/input";
 export default function LoginPage() {
   const router = useRouter();
   const { user, loading, login } = useAuth();
-  const [username, setUsername] = useState("admin");
+  const [username, setUsername] = useState("");
   const [password, setPassword] = useState("");
   const [submitting, setSubmitting] = useState(false);
   const [error, setError] = useState("");