feat: replace unified loading approach with global loading provider and refactor loading handling across components

- Introduced GlobalLoadingProvider to manage a consistent loading overlay across the application.
- Replaced existing loading implementations with useGlobalLoadingEffect hook for better control and to prevent animation resets.
- Updated components such as LoginPage, AuthCallbackPage, and DashboardLayout to utilize the new global loading mechanism.
- Removed UnifiedLoadingScreen component to streamline loading management and enhance user experience.

surfsense_web/app/auth/callback/loading.tsx

@@ -1,21 +1,14 @@
 "use client";
 
 import { useTranslations } from "next-intl";
-import { Spinner } from "@/components/ui/spinner";
+import { useGlobalLoadingEffect } from "@/hooks/use-global-loading";
 
 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>
-	);
-}
 
+	// Use global loading - spinner animation won't reset when page transitions
+	useGlobalLoadingEffect(true, t("processing_authentication"), "default");
+
+	// Return null - the GlobalLoadingProvider handles the loading UI
+	return null;
+}