Added goal icon support and tests

[ejlalshah]

Summary

This pull request adds support for goal icons and persistence across the frontend and backend.

Changes

• Updated the goal model to support icons.
• Added emoji picker functionality in GoalManager.
• Added PUT request support for updating goals.
• Implemented emoji selection persistence.
• Added unit tests for the GetForUser route in GoalControllerTests.
• Verified all tests pass successfully.

Testing

• Ran backend unit tests using xUnit.
• Verified icon changes persist after refresh.
• Confirmed frontend and backend integration works correctly.