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/atoms/ui/loading.atoms.ts

@@ -0,0 +1,31 @@
+import { atom } from "jotai";
+
+interface GlobalLoadingState {
+	isLoading: boolean;
+	message?: string;
+	variant: "login" | "default";
+}
+
+export const globalLoadingAtom = atom<GlobalLoadingState>({
+	isLoading: false,
+	message: undefined,
+	variant: "default",
+});
+
+// Helper atom for showing global loading
+export const showGlobalLoadingAtom = atom(
+	null,
+	(
+		get,
+		set,
+		{ message, variant = "default" }: { message?: string; variant?: "login" | "default" }
+	) => {
+		set(globalLoadingAtom, { isLoading: true, message, variant });
+	}
+);
+
+// Helper atom for hiding global loading
+export const hideGlobalLoadingAtom = atom(null, (get, set) => {
+	set(globalLoadingAtom, { isLoading: false, message: undefined, variant: "default" });
+});
+