Fix board choice overlays and attack splitting

[matthewevans]

No description provided.

[gemini-code-assist]

Code Review

This pull request refactors the collapsed group picker to render via a React Portal with dynamic positioning, and updates the attack target picker to distribute attackers globally when performing an even split. Additionally, it elevates the board layout's z-index when a board choice is active. Feedback highlights a potential issue where this elevated z-index (z-[45]) could obscure the reconnecting banner (z-40) during a disconnection, and suggests conditionally resetting the z-index when reconnecting.

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