refactor(layout): switch to document scroll with sticky sidebars

- Remove fixed container heights from three-pane layout
- Desktop: sticky sidebars with max-height, document scrolls
- Mobile: keep fixed overlays unchanged
- Update scroll direction hook to use window scroll
- Update progress indicator z-index to 1102 (above mobile overlays)
- Apply Tailwind utilities to App container
- Maintain responsive behavior across breakpoints

src/components/ThreePaneLayout.tsx

@@ -98,11 +98,10 @@ const ThreePaneLayout: React.FC<ThreePaneLayoutProps> = (props) => {
   const mainPaneRef = useRef<HTMLDivElement>(null)
   
   // Detect scroll direction to hide/show mobile buttons
-  // On mobile, scroll happens in the main pane, not on window
+  // Now using window scroll (document scroll) instead of pane scroll
   const scrollDirection = useScrollDirection({ 
     threshold: 10, 
-    enabled: isMobile && !props.isSidebarOpen && props.isHighlightsCollapsed,
-    elementRef: mainPaneRef
+    enabled: isMobile && !props.isSidebarOpen && props.isHighlightsCollapsed
   })
   const showMobileButtons = scrollDirection !== 'down'