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/base.css

@@ -84,8 +84,6 @@ body.modal-open {
 }
 
 .container {
-    max-width: 1200px;
-    margin: 0 auto;
     padding: 20px;
 }