[#5] Fixed pairs UI in-game

[javaisbetterthanpython]

Summary

• Add fixed pair link controls to PlayersModal (partner dropdown per player, link indicator)
• Extend editPlayers to accept and persist fixedPairs; pair changes force round regeneration
• Add shared fixedPairs helpers and PlayerPairSelect component
• Wire fixedPairs through state, localStorage, and worker message payload

Test plan

• yarn test:ci passes
• yarn build passes
• Open Players modal during a game, link two players, submit — current round regenerates
• Remove a paired player — pair association is cleared
• Cannot select self as partner (excluded from dropdown)

Closes #5

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