Add goal icon feature with emoji picker

[Seifudinsec]

Implements emoji selection for goals using emoji-mart v3.0.1.

Implements emoji selection for goals using emoji-mart v3.0.1, allowing users to personalize their goal cards with icons.

Changes

Backend
Added Icon field to Goal model in CommBank-Server/CommBank-Server/Models/Goal.cs

Frontend

1. Updated Goal interface to include icon: string | null field in src/api/types.ts
2. Created EmojiPicker component using emoji-mart v3.0.1 in src/ui/components/EmojiPicker.tsx
3. Integrated emoji picker in GoalManager with add/remove icon functionality in src/ui/features/goalmanager/GoalManager.tsx
4. Display icon on goal card in src/ui/pages/Main/goals/GoalCard.tsx

Testing

1. Added unit test for GetForUser endpoint in CommBank-Server/CommBank.Tests/GoalControllerTests.cs
2. All 11 tests passing

Manual Testing

1. Open in development mode
2. Click a goal card to open the modal
3. Click "Add icon" button to open emoji picker
4. Select an emoji - it saves immediately
5. Refresh browser - emoji persists on goal card
6. Click existing icon to remove it