Fix bottom margin when rendering in mobile (#3043)

## Summary:
Fix bottom margin when rendering in mobile

| Before | After |
| ------ | ----- |
| <img width="382" height="780" alt="image" src="https://github.com/user-attachments/assets/cdc38d5d-779e-4b13-a66b-3135abfb7cb4" /> | <img width="382" height="780" alt="image" src="https://github.com/user-attachments/assets/46260cd8-762c-425e-81f4-722c649a831f" /> |


Issue: LEMS-3517

## Test plan:
1. Navigate to an exercise with image like `internal-courses/test-everything/test-everything-1/te-image/e/image--numeric-input-exercise` and confirm that the bottom of the perseus renderer has space
2. Check other exercise and make sure other rendered items has proper spacing

Author: ivyolamit

Reviewers: catandthemachines, nishasy

Required Reviewers:

Approved By: catandthemachines, nishasy

Checks: ⏭️  1 check has been skipped, ✅ 10 checks were successful

Pull Request URL: #3043

Author: ivyolamit

.changeset/tall-berries-visit.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/perseus": patch
+---
+
+Fix bottom margin when rendering in mobile

packages/perseus/src/styles/styles.css

@@ -283,7 +283,8 @@
 .framework-perseus.perseus-mobile
     .perseus-renderer
     > .paragraph:not(:first-child) {
-    margin-top: 32px;
+    margin-top: var(--wb-sizing-size_240);
+    margin-bottom: var(--wb-sizing-size_240);
 }
 .framework-perseus.perseus-mobile .perseus-renderer > .paragraph > .paragraph {
     margin: 0;