- Add left navigation from hubs to sidebar when at leftmost item
- Add up navigation from first hub to tab bar (Libraries) or hero section (Discover)
- Enhance HubSection with onNavigateLeft and onNavigateUp callbacks
- Update _handleVerticalNavigation to return false at top boundary for proper UP handling
- Implement _navigateToSidebar() in Library Recommended tab and Discover screen
- Update all HubSection instances with proper navigation callbacks
- Use Dialog + Scaffold for TV to improve library visibility and keyboard navigation
- Added _ensureFocusedVisible() to scroll focused library into view on D-pad movement
- Maintained DraggableScrollableSheet for non-TV platforms (unchanged behavior)
- Only affects Android TV; Windows/desktop/mobile use original modal bottomsheet
- Preserves all existing functionality (reordering, visibility toggle, options menu)
dwk001