City combat refactor

[Rewenix]

Summary

This PR refactors several features that were developed earlier on the city_combat branch, with the goal of making the existing city combat and client UI behavior easier to reason about.

Changes

• Refactored the pure combat resolver to keep city state in mutable city snapshots instead of parallel HP/owner maps.
• Simplified city melee resolution and removed an unnecessary too_many_arguments allowance.
• Updated combat system comments to better describe the city HP, ownership, and movement resolution flow.
• Moved client city selection into UiState::CitySelected instead of storing it separately in Controller.
• Added UiState::GameFinished { outcome } so win/loss UI and input blocking are driven by one local UI state.
• Simplified UI systems by removing repeated DefeatedPlayer / VictoriousPlayer checks from action bars, production controls, turn text, and lobby start handling.
• Prevented map click handling from running when the mouse is over UI buttons, fixing action/production button clicks being swallowed by handle_left_click.
• Adjusted lobby visibility so defeated players do not make the lobby appear before the active game has ended.

Testing

• cargo check -p client
• cargo clippy -p client --all-targets