feat: add core terminations functionality

[aaronlee777]

This pull request introduces a new employee termination flow, including a new EmployeeTerminations component, supporting presentation, tests, i18n, and event constants. The changes add a complete UI for terminating employees, form validation, event emission, and integration with the API.

Key changes:

New Employee Terminations Feature:

• Added the EmployeeTerminations container component, which handles employee fetching, form state, validation, API mutation, and event emission for termination actions. (src/components/Terminations/EmployeeTerminations.tsx)
• Implemented the EmployeeTerminationsPresentation component to render the termination form UI, including last day selection, payroll options, error handling, and action buttons. (src/components/Terminations/EmployeeTerminationsPresentation.tsx)
• Created a comprehensive test suite for the new component, covering rendering, form validation, user interactions, and event emission. (src/components/Terminations/EmployeeTerminations.test.tsx)

Integration and Exports:

• Exported the new component from the Terminations module and added Terminations to the main components index for usage throughout the app. (src/components/Terminations/index.tsx, src/components/index.ts) [1] [2]

Internationalization and Constants:

• Added English i18n strings for the termination flow, including all form labels, descriptions, actions, and validation messages. (src/i18n/en/Terminations.EmployeeTerminations.json)
• Introduced new event constants for employee termination actions and integrated them into the shared componentEvents object. (src/shared/constants.ts) [1] [2]