Enhance signin flow: add toast notifications for error handling and update background color variable

app/styles/_variables.scss

@@ -1,5 +1,5 @@
 $color-accent: rgb(255, 69, 58);
-$color-background: #1a202c;
+$color-background: oklch(14.1% 0.005 285.823); // #1a202c
 $color-text: rgba(255, 255, 255, 0.92);
 $color-placeholder-text: rgba(255, 255, 255, 0.44);
 $color-border: rgba(255, 255, 255, 0.16);