refactor(navbar): reorder right side to Subscribe → Star → Support → GitHub

Promote the newsletter conversion to leftmost position in the right-side group so it gets the most visual weight without being pushy. The previous order (Support → Star → Subscribe → GitHub) buried Subscribe in the middle.

Order rationale (low-friction → high-friction → dev tooling):

  Subscribe — primary engagement, opens modal (#subscribe), fastest path to "stay in the loop"
  Star      — 1-click GitHub interaction, zero friction once signed in
  Support   — financial commitment, post-engagement CTA
  GitHub ▼  — developer dropdown (Discussions, Edit, Report, Source), sits next to Quarto's theme toggle + search tools

This is the source of truth for every Quarto site that includes navbar-common.yml via metadata-files. StaffML (Next.js) has its own header component and needs a parallel change there if Subscribe should appear in its navbar too — flagged separately.

shared/config/navbar-common.yml

@@ -149,18 +149,23 @@ website:
             text: "License"
             href: https://mlsysbook.ai/about/license.html
     right:
-      - icon: heart
-        text: "Support"
-        href: https://opencollective.com/mlsysbook
-        target: _blank
-      - icon: star
-        text: "Star"
-        href: https://github.com/harvard-edge/cs249r_book
-        target: _blank
+      # Right-side order: low-friction conversions first, dev tooling last.
+      #   Subscribe — primary engagement (modal, fastest path to "stay in the loop")
+      #   Star      — 1-click GitHub interaction, social proof
+      #   Support   — financial commitment, post-engagement CTA
+      #   GitHub ▼  — developer dropdown, sits next to Quarto's theme + search tools
       - icon: envelope
         text: "Subscribe"
         href: "#subscribe"
         id: "navbar-subscribe-btn"
+      - icon: star
+        text: "Star"
+        href: https://github.com/harvard-edge/cs249r_book
+        target: _blank
+      - icon: heart
+        text: "Support"
+        href: https://opencollective.com/mlsysbook
+        target: _blank
       - icon: github
         text: "GitHub"
         menu: