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/components/ui/Button.tsx

@@ -0,0 +1,54 @@
+import { forwardRef } from 'react';
+import { motion, HTMLMotionProps } from 'framer-motion';
+import { cn } from '../../lib/utils';
+import { Loader2 } from 'lucide-react';
+
+export type ButtonVariant = 'primary' | 'secondary' | 'ghost' | 'danger' | 'outline';
+export type ButtonSize = 'sm' | 'md' | 'lg';
+
+export interface ButtonProps extends Omit<HTMLMotionProps<'button'>, 'children'> {
+  variant?: ButtonVariant;
+  size?: ButtonSize;
+  loading?: boolean;
+  children?: React.ReactNode;
+}
+
+const variantStyles: Record<ButtonVariant, string> = {
+  primary: 'bg-primary text-white hover:bg-primary-hover',
+  secondary: 'bg-gray-100 text-gray-900 hover:bg-gray-200 dark:bg-gray-700 dark:text-gray-100 dark:hover:bg-gray-600',
+  ghost: 'text-gray-600 hover:bg-gray-100 dark:text-gray-300 dark:hover:bg-gray-700',
+  danger: 'bg-red-500 text-white hover:bg-red-600',
+  outline: 'border border-gray-300 text-gray-700 hover:bg-gray-50 dark:border-gray-600 dark:text-gray-300 dark:hover:bg-gray-800',
+};
+
+const sizeStyles: Record<ButtonSize, string> = {
+  sm: 'px-3 py-1.5 text-xs rounded-md',
+  md: 'px-4 py-2 text-sm rounded-lg',
+  lg: 'px-6 py-3 text-base rounded-lg',
+};
+
+export const Button = forwardRef<HTMLButtonElement, ButtonProps>(
+  ({ className, variant = 'primary', size = 'md', loading, disabled, children, ...props }, ref) => {
+    return (
+      <motion.button
+        ref={ref}
+        whileTap={{ scale: 0.98 }}
+        className={cn(
+          'inline-flex items-center justify-center font-medium transition-colors duration-fast',
+          'focus:outline-none focus-visible:ring-2 focus-visible:ring-primary focus-visible:ring-offset-2',
+          'disabled:opacity-50 disabled:cursor-not-allowed',
+          variantStyles[variant],
+          sizeStyles[size],
+          className
+        )}
+        disabled={disabled || loading}
+        {...props}
+      >
+        {loading && <Loader2 className="w-4 h-4 mr-2 animate-spin" />}
+        {children}
+      </motion.button>
+    );
+  }
+);
+
+Button.displayName = 'Button';