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/js/app.js

@@ -524,13 +524,13 @@ document.addEventListener('DOMContentLoaded', async () => {
     // Show content now that translations are loaded and tabs are set
     document.body.style.visibility = 'visible';
 
-    // Set CSS variable for sticky header height so section headers stack below it
+    // Set CSS variable for sticky header height (header now includes tab bar)
     const headerEl = document.querySelector('header');
     if (headerEl) {
         const updateHeaderHeight = () => {
-            document.documentElement.style.setProperty(
-                '--header-height', headerEl.offsetHeight + 'px'
-            );
+            const hh = headerEl.offsetHeight;
+            document.documentElement.style.setProperty('--header-height', hh + 'px');
+            document.documentElement.style.setProperty('--sticky-top', hh + 'px');
         };
         updateHeaderHeight();
         window.addEventListener('resize', updateHeaderHeight);