Update README with LeetCode Daily Challenge

Author: jeremymanning

README.md

@@ -396,6 +396,7 @@ _**Note:** Our group is currently taking a break from grinding leetcode. The tab
 | July 18, 2025 | [2163](https://leetcode.com/problems/minimum-difference-in-sums-after-removal-of-elements/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/2163) | 🔴 Hard |
 | July 19, 2025 | [1233](https://leetcode.com/problems/remove-sub-folders-from-the-filesystem/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/1233) | 🟡 Medium |
 | July 20, 2025 | [1948](https://leetcode.com/problems/delete-duplicate-folders-in-system/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/1948) | 🔴 Hard |
+| July 21, 2025 | [1957](https://leetcode.com/problems/delete-characters-to-make-fancy-string/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/1957) | 🟢 Easy |
 
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