feat(ui): enhance UI with animations, dark mode support and and improved components

- Add framer-motion page transitions and AnimatePresence support
- Add dark mode support across all components
- Create reusable UI components (Button, Badge, Card, EmptyState, Input, Toast, Skeleton)
- Add CSS custom properties for consistent theming
- Add animation variants and utility functions
- Improve ChatArea, Sidebar, TriggersPanel with animations

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/src/lib/utils.ts

@@ -0,0 +1,6 @@
+import { clsx, type ClassValue } from 'clsx';
+import { twMerge } from 'tailwind-merge';
+
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs));
+}