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/loading.tsx

@@ -0,0 +1,37 @@
+import { Loader2 } from 'lucide-react';
+import { cn } from '@/lib/utils';
+
+interface LoadingProps {
+  className?: string;
+  text?: string;
+  size?: 'sm' | 'md' | 'lg';
+}
+
+export function Loading({ className, text, size = 'md' }: LoadingProps) {
+  const sizeClasses = {
+    sm: 'h-4 w-4',
+    md: 'h-6 w-6',
+    lg: 'h-8 w-8',
+  };
+
+  return (
+    <div className={cn('flex items-center justify-center', className)}>
+      <Loader2 className={cn(sizeClasses[size], 'animate-spin mr-2')} />
+      {text && <span className="text-muted-foreground">{text}</span>}
+    </div>
+  );
+}
+
+// Full page loading component
+export function LoadingPage({ text = 'Loading...' }: { text?: string }) {
+  return (
+    <div className="flex min-h-[400px] items-center justify-center">
+      <Loading text={text} size="lg" />
+    </div>
+  );
+}
+
+// Inline loading for buttons or small areas
+export function LoadingInline({ text }: { text?: string }) {
+  return <Loading text={text} size="sm" />;
+}