Add icon support to Goal model, controller, and tests

[Pandhari-404]

Changes Made

• Added an optional Icon field (string) to the Goal model to support emoji icons
• Verified the API correctly returns and accepts the new icon field via Postman testing
• Added a unit test (GetForUser) to cover the GetGoalsForUser route in GoalControllerTests.cs
• Removed Secrets.json from version control and added it to .gitignore to protect sensitive credentials

Testing

• All 11 unit tests pass (dotnet test)
• Manually verified via Postman that the Goal API returns the icon field correctly