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>
+	);
+}
+

surfsense_web/app/auth/callback/page.tsx

@@ -1,23 +1,20 @@
+"use client";
+
 import { Suspense } from "react";
+import { useTranslations } from "next-intl";
+import { UnifiedLoadingScreen } from "@/components/ui/unified-loading-screen";
 import TokenHandler from "@/components/TokenHandler";
 
 export default function AuthCallbackPage() {
+	const t = useTranslations("auth");
+	
 	return (
-		<div className="container mx-auto p-4">
-			<h1 className="text-2xl font-bold mb-4">Authentication Callback</h1>
-			<Suspense
-				fallback={
-					<div className="flex items-center justify-center min-h-[200px]">
-						<div className="h-8 w-8 animate-spin rounded-full border-4 border-primary border-t-transparent"></div>
-					</div>
-				}
-			>
-				<TokenHandler
-					redirectPath="/dashboard"
-					tokenParamName="token"
-					storageKey="surfsense_bearer_token"
-				/>
-			</Suspense>
-		</div>
+		<Suspense fallback={<UnifiedLoadingScreen variant="default" message={t("processing_authentication")} />}>
+			<TokenHandler
+				redirectPath="/dashboard"
+				tokenParamName="token"
+				storageKey="surfsense_bearer_token"
+			/>
+		</Suspense>
 	);
 }