feat(ui): implement proper responsive navbar behavior

- Move all nav items to collapse into hamburger menu on mobile (≤767px)
- Change collapse-below to 'md' for better mobile experience
- Add text labels for better hamburger menu UX (⭐ Star on GitHub, etc.)
- Style hamburger menu with proper spacing, borders, and hover effects
- Desktop shows icons only, mobile shows descriptive text in menu
- Remove all hiding CSS - now uses proper Bootstrap collapse behavior

book/config/_quarto-html.yml

@@ -65,9 +65,9 @@ website:
     search: true
     pinned: true
     collapse: true
-    collapse-below: "lg"
+    collapse-below: "md"
     title: "ML Systems"
-    right:
+    left:
       - icon: code
         text: "Labs"
         href: contents/labs/labs.qmd
@@ -78,14 +78,14 @@ website:
         text: "PDF"
         href: https://mlsysbook.ai/pdf
         target: _blank
-    tools:
       - icon: star
+        text: "⭐ Star on GitHub"
         href: https://github.com/harvard-edge/cs249r_book
-        aria-label: "Star on GitHub"
       - icon: heart
+        text: "❤️ Donate & Support"
         href: https://opencollective.com/mlsysbook
-        aria-label: "Donate & Support"
       - icon: github
+        text: "GitHub Tools"
         menu:
           - text: "✏️ Edit this page"
             href: https://github.com/harvard-edge/cs249r_book