perf: change image type from jpg to webp

initially the login page background image size was 5.3MB and was of very high resolution. so i reduce the size by changing its file type from jpg to webp
and also decreasing its resolution by 50%. it makes a significant improvement in file size of image which is now 433KB.

this increases the performance.

Fixes: #1296

src/app/login/login.component.scss

@@ -1,5 +1,5 @@
 .cover {
-  background: url("../../assets/images/cover_image.jpg") no-repeat center center;
+  background: url("../../assets/images/cover_image_resized.webp") no-repeat center center;
   background-size: cover;
   position: relative;
   z-index: 0;