feat: establish core backend API server with S2A proxy, configuration, and mail services, alongside frontend email configuration page and toast component.

frontend/src/components/common/Toast.tsx

@@ -78,7 +78,7 @@ function ToastContainer({ toasts, onRemove }: ToastContainerProps) {
     if (toasts.length === 0) return null
 
     return (
-        <div className="fixed bottom-6 right-6 z-[9999] space-y-3">
+        <div className="fixed top-6 right-6 z-[9999] space-y-3">
             {toasts.map((toast) => (
                 <ToastItem key={toast.id} toast={toast} onRemove={onRemove} />
             ))}