Add Leo Lee to CSS Mixins discussion (#35)

Co-authored-by: Kevin Babbitt <kbabbitt@microsoft.com>

Author: leotlee

planning/berlin-2026/index.md

@@ -80,6 +80,7 @@ Please list your availability in Berlin time (morning/afternoon/all or specific
 | Hoch Hochkeppel | PDT | 16:00-end | |
 | Celeste Pan | PDT | 16:00-end | maybe earlier if necessary |
 | Yanling Wang | PDT | 16:00-end | |
+| Leo Lee | PDT | 16:00-end | interested in attending CSS Mixins discussion|
 | Stephanie Zhang | PDT | 16:00-end | interested in attending the CSS Mixins discussion |
 
 ### Regrets