fix(mobile): correct initial height for sign in layout (#12380)

packages/frontend/core/src/mobile/components/sign-in/layout.css.ts

@@ -1,11 +1,11 @@
-import { style } from '@vanilla-extract/css';
+import { fallbackVar, style } from '@vanilla-extract/css';
 
 import { globalVars } from '../../styles/variables.css';
 
 export const root = style({
   padding: '40px',
   justifyContent: 'flex-end',
-  minHeight: `calc(100dvh - ${globalVars.appKeyboardHeight})`,
+  minHeight: `calc(100dvh - ${fallbackVar(globalVars.appKeyboardHeight, '0px')})`,
   display: 'flex',
   flexDirection: 'column',
   position: 'relative',