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/features/targets.js

@@ -723,6 +723,7 @@ export async function loadTargetsTab() {
             // Render tree sidebar with expand/collapse buttons
             _targetsTree.setExtraHtml(`<button class="btn-expand-collapse" onclick="expandAllTargetSections()" data-i18n-title="section.expand_all" title="${t('section.expand_all')}">⊞</button><button class="btn-expand-collapse" onclick="collapseAllTargetSections()" data-i18n-title="section.collapse_all" title="${t('section.collapse_all')}">⊟</button><button class="tutorial-trigger-btn" onclick="startTargetsTutorial()" data-i18n-title="tour.restart" title="${t('tour.restart')}">${ICON_HELP}</button>`);
             _targetsTree.update(treeGroups, activeLeaf);
+            _targetsTree.observeSections('targets-panel-content');
         }
 
         // Show/hide stop-all buttons based on running state