Add support for goal icons via emoji picker

[anshikasri8304]

Summary

Adds the ability for users to add and change an icon (emoji) on their goals, with changes persisting via the backend.

Changes

• Added optional icon field to the Goal model (src/api/types.ts)
• GoalCard now displays the goal's icon when one is set
• GoalManager integrates the emoji-mart picker (v3.0.1):
  • Shows an "Add icon" button when no icon is set
  • Shows the current icon (click to change it) when one is set
• pickEmojiOnClick updates Redux state and calls the updateGoal PUT request, so icon changes persist after a client refresh

Testing

Verified via Postman that PUT requests to /api/Goal/{goalId} with an updated icon persist correctly and are reflected after a client refresh.