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/streams.js

@@ -1538,6 +1538,14 @@ function renderPictureSourcesList(streams) {
         // Render tree sidebar with expand/collapse buttons
         _streamsTree.setExtraHtml(`<button class="btn-expand-collapse" onclick="expandAllStreamSections()" data-i18n-title="section.expand_all" title="${t('section.expand_all')}">⊞</button><button class="btn-expand-collapse" onclick="collapseAllStreamSections()" data-i18n-title="section.collapse_all" title="${t('section.collapse_all')}">⊟</button><button class="tutorial-trigger-btn" onclick="startSourcesTutorial()" data-i18n-title="tour.restart" title="${t('tour.restart')}">${ICON_HELP}</button>`);
         _streamsTree.update(treeGroups, activeTab);
+        _streamsTree.observeSections('streams-list', {
+            'raw-streams': 'raw', 'raw-templates': 'raw',
+            'static-streams': 'static_image',
+            'proc-streams': 'processed', 'proc-templates': 'processed',
+            'audio-multi': 'audio', 'audio-mono': 'audio', 'audio-templates': 'audio',
+            'value-sources': 'value',
+            'sync-clocks': 'sync',
+        });
     }
 }