feat: implement unified loading screens across various components

- Introduced a new UnifiedLoadingScreen component for consistent loading indicators in the application.
- Replaced existing loading implementations in LoginPage, AuthCallbackPage, DashboardLayout, and other components with the new unified loading screen.
- Updated translations for loading messages to enhance user experience and clarity.
- Improved loading states in the ElectricProvider and TokenHandler components to utilize the new loading screen, ensuring a cohesive look and feel during loading processes.

surfsense_web/app/auth/callback/loading.tsx

@@ -0,0 +1,19 @@
+import { useTranslations } from "next-intl";
+import { Spinner } from "@/components/ui/spinner";
+
+export default function AuthCallbackLoading() {
+	const t = useTranslations("auth");
+	return (
+		<div className="fixed inset-0 z-[9999] flex min-h-screen flex-col items-center justify-center bg-background">
+			<div className="flex flex-col items-center space-y-4">
+				<div className="h-12 w-12 flex items-center justify-center">
+					<Spinner size="xl" className="text-primary" />
+				</div>
+				<span className="text-muted-foreground text-sm min-h-[1.25rem] text-center max-w-md px-4">
+					{t("processing_authentication")}
+				</span>
+			</div>
+		</div>
+	);
+}
+