Support icons

[HarshilPatel69]

Goals felt a bit plain, so this adds the ability to give each one an emoji icon.

What I did:

• Added an optional icon field to the Goal model.
• Each goal card now shows its icon up top, so they're easy to tell apart at a glance.
• Pulled in emoji-mart (v3.0.1) for the picker and wrapped it so it follows the
  app's light/dark theme.
• In the goal manager, if a goal has no icon yet you'll see an "Add icon" button;
  if it already has one, you can just click it to swap it out. Picking an emoji
  updates the UI right away, saves it to the store, and writes it back to the
  backend so it sticks.