Add icon support to Goal model, GoalCard, and GoalManager

[Pandhari-404]

Changes Made

• Added an optional icon field (string) to the Goal interface in types.ts
• Displayed the icon on the GoalCard component
• Implemented an emoji picker in GoalManager.tsx allowing users to add/change a goal's icon
• Wired up the PUT request so icon changes persist to the backend and survive page refresh

Testing

• Manually verified in the browser that icons display on goal cards and persist after refresh