docs: add sticky header support for documentation pages

- Add CSS rules to web/static/input.css for header positioning
- Header is sticky only on /docs pages via .page-docs body class
- Add page-docs class to body element in templates/docs.templ
- Non-docs pages have static header position

web/static/input.css

@@ -144,6 +144,24 @@
     color: var(--text-primary);
   }
 
+  /* Header component styles */
+  .header-nav {
+    position: sticky;
+    top: 0;
+  }
+
+  /* Make header sticky only on docs pages */
+  body:not(.page-docs) .header-nav {
+    position: static;
+  }
+
+  /* Add class to body on docs pages */
+  .page-docs .header-nav {
+    position: sticky;
+    top: 0;
+    z-index: 50;
+  }
+
   .card {
     background-color: var(--bg-secondary);
     border-color: var(--border);