Fix primary sidebar animations when secondary sidebar is open

[everyoneexe]

• Add missing initExplorerHandler() call in updateHandler.ts
• Fix Smooth-Mode.scss selectors to work with both sidebars
• Add comprehensive sidebarHandler.ts for all panel states
• Add terminal toggle animations with slide effects
• Add extension panel toggle animations
• Support all sidebar combinations (both-sidebars-open, etc.)
• Enhanced CSS timing functions for smooth UX