feat(frontend): improve error handling and UX - Sprint 3 Story 4

Add comprehensive error handling with Error Boundary and improve user feedback.

Changes:
- Created global ErrorBoundary component with fallback UI using react-error-boundary
- Integrated ErrorBoundary in root layout to catch all errors
- Created Loading component with variants (sm, md, lg) for consistent loading states
- Created EmptyState component for better empty data display with CTAs
- Improved form error messages in login and register pages (consistent destructive styling)
- Updated projects page to use EmptyState component
- Added better error handling with retry actions

UX improvements:
- Better error messages and recovery options with clear action buttons
- Consistent loading indicators across all pages
- Helpful empty states with clear descriptions and CTAs
- Graceful error handling without crashes
- Consistent destructive color theme for all error messages

Technical:
- Installed react-error-boundary package (v5)
- All TypeScript types are properly defined
- Build and type checking pass successfully

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

components/ui/empty-state.tsx

@@ -0,0 +1,44 @@
+import { LucideIcon } from 'lucide-react';
+import { Button } from '@/components/ui/button';
+import { cn } from '@/lib/utils';
+
+interface EmptyStateProps {
+  icon: LucideIcon;
+  title: string;
+  description: string;
+  action?: {
+    label: string;
+    onClick: () => void;
+    variant?: 'default' | 'outline' | 'secondary';
+  };
+  className?: string;
+}
+
+export function EmptyState({
+  icon: Icon,
+  title,
+  description,
+  action,
+  className
+}: EmptyStateProps) {
+  return (
+    <div className={cn(
+      'flex flex-col items-center justify-center py-12 px-4',
+      className
+    )}>
+      <Icon className="h-16 w-16 text-muted-foreground/50 mb-4" />
+      <h3 className="text-lg font-semibold mb-2">{title}</h3>
+      <p className="text-muted-foreground text-center mb-6 max-w-sm">
+        {description}
+      </p>
+      {action && (
+        <Button
+          onClick={action.onClick}
+          variant={action.variant || 'default'}
+        >
+          {action.label}
+        </Button>
+      )}
+    </div>
+  );
+}