Support icons

[saipyae2004]

Summary

Adds support for goal icons across the client.

Changes

• Added an optional icon field to the Goal type (src/api/types.ts)
• Display the goal's icon on each goal card (GoalCard.tsx)
• Integrated the emoji-mart emoji picker
• Updated GoalManager.tsx so users can add and change a goal's icon
• Persist icon changes via the updateGoal PUT request (src/api/lib.ts)

Testing

• Icons display on goal cards and the emoji picker opens
• Icon changes persist after a page refresh