Add Icon field to Goal model and unit tests for GetForUser

[Ezaj24]

Changes Made

Backend (Task 1)

• Added optional Icon field to Goal model (Goal.cs)

Testing (Task 4)

• Completed GetForUser unit test in GoalControllerTests.cs
• All 11 tests passing

Purpose

This PR adds icon support to the Goal model and ensures
test coverage for the GetForUser controller route.

[gitguardian]

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