fix: mobile device minimum width (#1337)

frontend/src/styles/common-imports.scss

@@ -15,8 +15,8 @@ $family-sans-serif: 'Open Sans', Helvetica, Arial, sans-serif;
 // the default values get overwritten by the definitions above
 @import "bulma-css-variables/sass/utilities/_all";
 
-// since $tablet is defined by bulma we can just define it after importing the utilities
-$mobile: math.div($tablet, 2);
+// mobile minimum width
+$mobile: 320px;
 
 $vikunja-font: 'Quicksand', sans-serif;