Add goal icon support across application

[raghavthaman]

Implemented support for goal icons.

Changes:

• Added optional Icon property to Goal backend model.
• Updated frontend Goal model to support icons.
• Added emoji picker support.
• Displayed icons on goal cards.
• Added PUT request to persist icon changes.
• Added xUnit tests covering GoalController icon support.

Testing:

• Verified goals load successfully.
• Verified icon changes persist after refresh.
• Verified backend tests pass.

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