Add btn-icon class to navbar toggler

Add the btn-icon utility class to the navbar toggler button in the navbar-fixed example to apply icon button styling. This change updates site/src/assets/examples/navbar-fixed/index.astro and keeps the existing SVG toggler markup and collapse behavior unchanged.

Author: coliff

site/src/assets/examples/navbar-fixed/index.astro

@@ -8,7 +8,7 @@ export const extra_css = ['navbar-fixed.css']
 <nav class="navbar md:navbar-expand navbar-dark fixed-top bg-dark">
   <div class="container-fluid">
     <a class="navbar-brand" href="#">Fixed navbar</a>
-    <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarCollapse" aria-controls="navbarCollapse" aria-expanded="false" aria-label="Toggle navigation">
+    <button class="btn-icon navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarCollapse" aria-controls="navbarCollapse" aria-expanded="false" aria-label="Toggle navigation">
       <svg class="navbar-toggler-icon" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 16 16" fill="none" stroke="currentColor" stroke-width="1" stroke-linecap="round"><path d="M1 3.5h14M1 8h14M1 12.5h14"/></svg>
     </button>
     <div class="collapse navbar-collapse" id="navbarCollapse">