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/streams.css

@@ -644,7 +644,7 @@
     cursor: pointer;
     user-select: none;
     position: sticky;
-    top: 0;
+    top: var(--header-height, 0px);
     z-index: 10;
     background: var(--bg-color);
     padding: 8px 0;