Added employee persistence, serialization, and file loading support

<SRS>SRS02 : Employee Management </SRS>

<Changes>
 - Implemented serialization and deserialization for Employee and GeneralEmployee models
 - Added FileManager integration to load employees from CSV files
 - Introduced ApplicationConfig entries for employee file paths
 - Updated Employee ID handling (getEmployeeId → getId) across project
 - Modified FileIO to auto-create file if not found instead of throwing exception
 - Added constructors for all employee types to support deserialization
 - Implemented loadEmployees in service and loadStates in controller
 - Ensured default admin creation if none exists during load
 - Added StringHelper utility for extracting numeric IDs
 - Extended Enums with string conversion and parsing utilities
 - Added initial CSV files for Employee and GeneralEmployee data
 - Improved login error message formatting and minor cleanup
 - Setup gitignore to not track csv files
</Changes>

.gitignore

@@ -426,3 +426,6 @@ FodyWeavers.xsd
 *.msix
 *.msm
 *.msp
+
+# CSV Files
+*.csv