Update README with LeetCode Daily Challenge

Author: jeremymanning

README.md

@@ -38,6 +38,7 @@ This means you'll always find an AI-generated solution to compare with or learn
 | March 16, 2026 | [1878](https://leetcode.com/problems/get-biggest-three-rhombus-sums-in-a-grid/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/1878) | 🟡 Medium |
 | March 17, 2026 | [1727](https://leetcode.com/problems/largest-submatrix-with-rearrangements/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/1727) | 🟡 Medium |
 | March 18, 2026 | [3070](https://leetcode.com/problems/count-submatrices-with-top-left-element-and-sum-less-than-k/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/3070) | 🟡 Medium |
+| March 19, 2026 | [3212](https://leetcode.com/problems/count-submatrices-with-equal-frequency-of-x-and-y/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/3212) | 🟡 Medium |
 
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