Mark mobile navigation task as completed in TODO.md

Author: jedisct1

www/TODO.md

@@ -16,7 +16,7 @@ This document outlines planned improvements and future tasks for the IPCrypt web
 
 - [x] Create a proper favicon and apple-touch-icon
 - [x] Add more visual elements to explain encryption modes
-- [ ] Improve mobile navigation for smaller screens
+- [x] Improve mobile navigation for smaller screens
 - [ ] Enhance print styles for better documentation printing
 
 ### Technical Implementation