Add goal icon selection and persistence

[Yannick-o1]

Summary

• add nullable icon support to the client Goal type
• display goal icons on dashboard cards
• let users add or change icons with the existing emoji picker
• update local state and Redux immediately after selection
• persist icon changes through the existing goal PUT request

Why

The frontend previously had no way to display, select, or persist goal icons.

User impact

Users can choose an emoji for a goal, see it on the goal card, and retain the change after refresh.

Validation

• npm run build
• development server compiled successfully and returned HTTP 200