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/ErrorBoundary.tsx

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+'use client';
+
+import { ErrorBoundary as ReactErrorBoundary } from 'react-error-boundary';
+import { AlertCircle } from 'lucide-react';
+import { Button } from '@/components/ui/button';
+
+interface ErrorFallbackProps {
+  error: Error;
+  resetErrorBoundary: () => void;
+}
+
+function ErrorFallback({ error, resetErrorBoundary }: ErrorFallbackProps) {
+  return (
+    <div className="flex min-h-[400px] flex-col items-center justify-center p-4">
+      <AlertCircle className="h-16 w-16 text-destructive mb-4" />
+      <h2 className="text-2xl font-bold mb-2">Something went wrong</h2>
+      <p className="text-muted-foreground mb-4 text-center max-w-md">
+        {error.message || 'An unexpected error occurred'}
+      </p>
+      <div className="flex gap-2">
+        <Button onClick={resetErrorBoundary}>
+          Try again
+        </Button>
+        <Button
+          variant="outline"
+          onClick={() => window.location.href = '/'}
+        >
+          Go to Dashboard
+        </Button>
+      </div>
+    </div>
+  );
+}
+
+interface ErrorBoundaryProps {
+  children: React.ReactNode;
+}
+
+export function ErrorBoundary({ children }: ErrorBoundaryProps) {
+  return (
+    <ReactErrorBoundary
+      FallbackComponent={ErrorFallback}
+      onReset={() => {
+        // Optional: Reset application state here
+        // For now, we'll just reload the current page
+        window.location.reload();
+      }}
+      onError={(error, errorInfo) => {
+        // Log error to console in development
+        console.error('Error caught by boundary:', error, errorInfo);
+
+        // In production, you could send this to an error tracking service
+        // like Sentry, LogRocket, etc.
+      }}
+    >
+      {children}
+    </ReactErrorBoundary>
+  );
+}

components/ui/empty-state.tsx

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+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>
+  );
+}

components/ui/loading.tsx

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+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" />;
+}