Sticky header, dim overlay on card navigation, fix sticky stacking

Make header sticky so search button stays visible on scroll. Section
headers stick below it using a JS-measured --header-height variable.
Add dim overlay behind highlighted cards for better focus effect.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/css/layout.css

@@ -2,10 +2,11 @@ header {
     display: flex;
     justify-content: space-between;
     align-items: center;
-    padding: 8px 0 6px;
-    margin-bottom: 6px;
-    position: relative;
+    padding: 8px 0 12px;
+    position: sticky;
+    top: 0;
     z-index: 100;
+    background: var(--bg-color);
 }
 
 .header-title {