refactor: Flatten directory structure

Move project files from uzdb/ subdirectory to root directory for cleaner project structure.

Changes:
- Move frontend/ to root
- Move internal/ to root
- Move build/ to root
- Move all config files (go.mod, wails.json, etc.) to root
- Remove redundant uzdb/ subdirectory nesting

Project structure is now:
├── frontend/        # React application
├── internal/        # Go backend
├── build/           # Wails build assets
├── doc/             # Design documentation
├── main.go          # Entry point
└── ...

🤖 Generated with Qoder

internal/middleware/error.go

@@ -0,0 +1,125 @@
+package middleware
+
+import (
+	"errors"
+	"net/http"
+	"runtime/debug"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"go.uber.org/zap"
+
+	"uzdb/internal/config"
+	"uzdb/internal/models"
+)
+
+// RecoveryMiddleware returns a recovery middleware that handles panics
+func RecoveryMiddleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		defer func() {
+			if err := recover(); err != nil {
+				logger := config.GetLogger()
+				
+				// Log the panic with stack trace
+				logger.Error("panic recovered",
+					zap.Any("error", err),
+					zap.String("stack", string(debug.Stack())),
+					zap.String("path", c.Request.URL.Path),
+					zap.String("method", c.Request.Method),
+				)
+
+				// Send error response
+				c.JSON(http.StatusInternalServerError, models.ErrorResponse{
+					Error:     "INTERNAL_ERROR",
+					Message:   "An unexpected error occurred",
+					Timestamp: time.Now(),
+					Path:      c.Request.URL.Path,
+				})
+				
+				c.Abort()
+			}
+		}()
+		
+		c.Next()
+	}
+}
+
+// ErrorMiddleware returns an error handling middleware
+func ErrorMiddleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.Next()
+		
+		// Check if there are any errors
+		if len(c.Errors) > 0 {
+			handleErrors(c)
+		}
+	}
+}
+
+// handleErrors processes and formats errors
+func handleErrors(c *gin.Context) {
+	logger := config.GetLogger()
+	
+	for _, e := range c.Errors {
+		err := e.Err
+		
+		// Log the error
+		logger.Error("request error",
+			zap.String("error", err.Error()),
+			zap.Any("type", e.Type),
+			zap.String("path", c.Request.URL.Path),
+		)
+		
+		// Determine status code
+		statusCode := http.StatusInternalServerError
+		
+		// Map specific errors to status codes
+		switch {
+		case errors.Is(err, models.ErrNotFound):
+			statusCode = http.StatusNotFound
+		case errors.Is(err, models.ErrValidationFailed):
+			statusCode = http.StatusBadRequest
+		case errors.Is(err, models.ErrUnauthorized):
+			statusCode = http.StatusUnauthorized
+		case errors.Is(err, models.ErrForbidden):
+			statusCode = http.StatusForbidden
+		case errors.Is(err, models.ErrConnectionFailed):
+			statusCode = http.StatusBadGateway
+		}
+		
+		// Send response if not already sent
+		if !c.Writer.Written() {
+			c.JSON(statusCode, models.ErrorResponse{
+				Error:     getErrorCode(err),
+				Message:   err.Error(),
+				Timestamp: time.Now(),
+				Path:      c.Request.URL.Path,
+			})
+		}
+		
+		// Only handle first error
+		break
+	}
+}
+
+// getErrorCode maps errors to error codes
+func getErrorCode(err error) string {
+	switch {
+	case errors.Is(err, models.ErrNotFound):
+		return string(models.CodeNotFound)
+	case errors.Is(err, models.ErrValidationFailed):
+		return string(models.CodeValidation)
+	case errors.Is(err, models.ErrUnauthorized):
+		return string(models.CodeUnauthorized)
+	case errors.Is(err, models.ErrForbidden):
+		return string(models.CodeForbidden)
+	case errors.Is(err, models.ErrConnectionFailed):
+		return string(models.CodeConnection)
+	case errors.Is(err, models.ErrQueryFailed):
+		return string(models.CodeQuery)
+	case errors.Is(err, models.ErrEncryptionFailed):
+		return string(models.CodeEncryption)
+	default:
+		return string(models.CodeInternal)
+	}
+}