Complete backend, frontend, and unit testing tasks

[thafshan]

Summary

• Added an optional icon field to the Goal model.
• Updated the frontend goal manager to allow users to select and save goal emojis.
• Connected frontend goal updates to the backend PUT endpoint.
• Added unit tests for the GoalController, including the GetForUser route.
• Added the backend response file required for the task.

Testing

• Ran dotnet test
• All tests passed successfully.

[gitguardian]

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