Fix header position shift when becoming sticky

Move header outside .container so it spans full viewport width
with its own padding, eliminating layout shift on scroll.

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

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

@@ -2,7 +2,7 @@ header {
     display: flex;
     justify-content: space-between;
     align-items: center;
-    padding: 8px 0 8px;
+    padding: 8px 20px;
     position: sticky;
     top: 0;
     z-index: 100;