feat: Add comprehensive backend validation, toast notifications, and Tokyo Night theme

- Backend: Add server-side validation with go-playground/validator/v10
- Frontend: Add toast notifications for API errors with @zerodevx/svelte-toast
- UI: Complete Tokyo Night theme redesign with modern animations
- Docs: Update COMPLETE_DOCUMENTATION.md and README.md with all enhancements
- Validation: Email format, password strength, and input sanitization
- UX: Real-time error feedback, loading states, and responsive design

backend/internal/database/connection.go

@@ -0,0 +1,20 @@
+package database
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+func NewConnection(databaseURL string) (*pgxpool.Pool, error) {
+	pool, err := pgxpool.New(context.Background(), databaseURL)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := pool.Ping(context.Background()); err != nil {
+		return nil, err
+	}
+
+	return pool, nil
+}