feat(ui): improve mobile navbar responsiveness and cleanup

- Fix mobile navbar collapse behavior and layout
- Resolve GitHub dropdown highlighting issue in hamburger menu
- Position search bar properly on mobile without white space
- Clean up TOC by disabling duplicate repo-actions
- Ensure all navigation items collapse into hamburger menu appropriately
- Style secondary tools (Star, Support, GitHub, PDF) in subdued grey
- Improve overall mobile user experience and visual consistency

book/config/_quarto-html.yml

@@ -101,7 +101,7 @@ website:
             href: https://github.com/harvard-edge/cs249r_book
 
   favicon: assets/images/icons/favicon.png
-
+  
   sidebar:
     - id: main-content
       title: "Main Content"
@@ -307,7 +307,6 @@ website:
 
   repo-url: https://github.com/harvard-edge/cs249r_book
   repo-branch: widget_quiz
-  repo-actions: [edit, issue, source]
 
   page-footer:
     left: |