feat: move theme toggle and GitHub button to top-right navbar

Remove socialLinks config and three-dots menu. Add appearance toggle
and combined GitHub + Star button directly in the navbar via layout
slot. The button shows the Octocat icon, "Star" label, and live
stargazer count.

apps/docs/.vitepress/theme/Layout.vue

@@ -1,5 +1,6 @@
 <script setup>
 import DefaultTheme from "vitepress/theme";
+import VPSwitchAppearance from "vitepress/dist/client/theme-default/components/VPSwitchAppearance.vue";
 import GitHubStars from "./GitHubStars.vue";
 
 const { Layout } = DefaultTheme;
@@ -8,7 +9,10 @@ const { Layout } = DefaultTheme;
 <template>
   <Layout>
     <template #nav-bar-content-after>
-      <GitHubStars />
+      <div class="nav-bar-right">
+        <VPSwitchAppearance />
+        <GitHubStars />
+      </div>
     </template>
   </Layout>
 </template>