feat(ui): improve mobile navbar responsiveness

- Hide title text on mobile (≤768px), show logo only
- Hide secondary tools (star, heart, GitHub) on mobile for cleaner header
- Maintain full desktop experience with all icons
- Optimize navbar spacing and touch targets for mobile
- Add responsive breakpoints for different screen sizes

book/config/_quarto-html.yml

@@ -65,8 +65,8 @@ website:
     search: true
     pinned: true
     collapse: true
-    collapse-below: "md"
-    title: "Machine Learning Systems"
+    collapse-below: "lg"
+    title: "ML Systems"
     right:
       - icon: code
         text: "Labs"