Fix duplicate React keys in EntitiesCard - #35

Summary

• Ensure each EntityCard in EntitiesCard gets a unique React key when entity ids are empty or duplicated (common with unfinished Puck array rows).

Test plan

• Open a dashboard page that uses an Entities Card
• Confirm the console no longer warns about missing/duplicate keys
• Add a new empty entity row in the editor and verify the warning still does not appear
• Confirm entity cards still render and toggle correctly

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