Sticky header with centered tabs, scroll-spy, and full-width layout

- Move tab bar into header (centered between title and toolbar)
- Make entire header sticky with border-bottom separator
- Remove container max-width for full-width layout
- Add scroll-spy: tree sidebar tracks visible section on scroll
- Remember scroll position per tab when switching
- Remove sticky section headers, use scroll-margin-top instead
- Update sticky offsets to use --sticky-top CSS variable

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

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

@@ -2,11 +2,12 @@ header {
     display: flex;
     justify-content: space-between;
     align-items: center;
-    padding: 8px 0 12px;
+    padding: 8px 0 8px;
     position: sticky;
     top: 0;
     z-index: 100;
     background: var(--bg-color);
+    border-bottom: 2px solid var(--border-color);
 }
 
 .header-title {
@@ -148,8 +149,6 @@ h2 {
     display: flex;
     align-items: center;
     gap: 4px;
-    border-bottom: 2px solid var(--border-color);
-    margin-bottom: 16px;
 }
 
 .tab-btn {