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/config/config.go

@@ -0,0 +1,42 @@
+package config
+
+import (
+	"fmt"
+	"os"
+)
+
+type Config struct {
+	ServerPort       string
+	JWTSecret        string
+	UploadPath       string
+	DatabaseHost     string
+	DatabasePort     string
+	DatabaseUser     string
+	DatabasePassword string
+	DatabaseName     string
+}
+
+func LoadConfig() *Config {
+	return &Config{
+		ServerPort:       getEnv("SERVER_PORT", "8080"),
+		DatabaseHost:     getEnv("DATABASE_HOST", "localhost"),
+		DatabasePort:     getEnv("DATABASE_PORT", "5432"),
+		DatabaseUser:     getEnv("DATABASE_USER", "postgres"),
+		DatabasePassword: getEnv("DATABASE_PASSWORD", "password"),
+		DatabaseName:     getEnv("DATABASE_NAME", "ebookdb"),
+		JWTSecret:        getEnv("JWT_SECRET", "your-secret-key"),
+		UploadPath:       getEnv("UPLOAD_PATH", "./uploads"),
+	}
+}
+
+func (c *Config) DatabaseURL() string {
+	return fmt.Sprintf("postgres://%s:%s@%s:%s/%s?sslmode=disable",
+		c.DatabaseUser, c.DatabasePassword, c.DatabaseHost, c.DatabasePort, c.DatabaseName)
+}
+
+func getEnv(key, defaultValue string) string {
+	if value := os.Getenv(key); value != "" {
+		return value
+	}
+	return defaultValue
+}