feat: Add optional Icon field to Goal model

[Waleed-Abdullah-Abbasi]

Summary

This PR adds an optional Icon field to the Goal model, which is the first step toward supporting emojis/icons for goals across the application.

Changes Made

• ✅ Added public string? Icon { get; set; } to the Goal class in Goal.cs
• ✅ Updated backend data models to support the new property
• ✅ No breaking changes – field is optional and nullable

Next Steps

• Backend PUT endpoint to update icons
• Frontend UI to pick emojis
• Unit tests for the GetForUser route

Checklist

• Code follows project conventions
• No breaking changes