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/streams.css

@@ -700,7 +700,7 @@ body.pp-filter-dragging .pp-filter-drag-handle {
 /* Sub-tab content sections */
 .subtab-section {
     margin-bottom: 24px;
-    scroll-margin-top: var(--header-height, 48px);
+    scroll-margin-top: var(--sticky-top, 90px);
 }
 
 .subtab-section:last-child {
@@ -728,10 +728,6 @@ body.pp-filter-dragging .pp-filter-drag-handle {
     gap: 8px;
     cursor: pointer;
     user-select: none;
-    position: sticky;
-    top: var(--header-height, 0px);
-    z-index: 10;
-    background: var(--bg-color);
     padding: 8px 0;
 }