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/models/errors.go

@@ -0,0 +1,74 @@
+package models
+
+import "errors"
+
+// Application errors
+var (
+	// ErrNotFound resource not found
+	ErrNotFound = errors.New("resource not found")
+	
+	// ErrAlreadyExists resource already exists
+	ErrAlreadyExists = errors.New("resource already exists")
+	
+	// ErrValidationFailed validation failed
+	ErrValidationFailed = errors.New("validation failed")
+	
+	// ErrUnauthorized unauthorized access
+	ErrUnauthorized = errors.New("unauthorized access")
+	
+	// ErrForbidden forbidden access
+	ErrForbidden = errors.New("forbidden access")
+	
+	// ErrInternalServer internal server error
+	ErrInternalServer = errors.New("internal server error")
+	
+	// ErrConnectionFailed connection failed
+	ErrConnectionFailed = errors.New("connection failed")
+	
+	// ErrQueryFailed query execution failed
+	ErrQueryFailed = errors.New("query execution failed")
+	
+	// ErrEncryptionFailed encryption/decryption failed
+	ErrEncryptionFailed = errors.New("encryption/decryption failed")
+	
+	// ErrInvalidConfig invalid configuration
+	ErrInvalidConfig = errors.New("invalid configuration")
+	
+	// ErrDatabaseLocked database is locked
+	ErrDatabaseLocked = errors.New("database is locked")
+	
+	// ErrTimeout operation timeout
+	ErrTimeout = errors.New("operation timeout")
+)
+
+// ErrorCode represents error codes for API responses
+type ErrorCode string
+
+const (
+	// CodeSuccess successful operation
+	CodeSuccess ErrorCode = "SUCCESS"
+	
+	// CodeNotFound resource not found
+	CodeNotFound ErrorCode = "NOT_FOUND"
+	
+	// CodeValidation validation error
+	CodeValidation ErrorCode = "VALIDATION_ERROR"
+	
+	// CodeUnauthorized unauthorized
+	CodeUnauthorized ErrorCode = "UNAUTHORIZED"
+	
+	// CodeForbidden forbidden
+	CodeForbidden ErrorCode = "FORBIDDEN"
+	
+	// CodeInternal internal error
+	CodeInternal ErrorCode = "INTERNAL_ERROR"
+	
+	// CodeConnection connection error
+	CodeConnection ErrorCode = "CONNECTION_ERROR"
+	
+	// CodeQuery query error
+	CodeQuery ErrorCode = "QUERY_ERROR"
+	
+	// CodeEncryption encryption error
+	CodeEncryption ErrorCode = "ENCRYPTION_ERROR"
+)