[#28] Fix blank rounds, re-jumble, and court-count bugs

[javaisbetterthanpython]

Summary

• Fix blank round view after round 1 by keeping the current round until regeneration succeeds and syncing round index after new rounds
• Handle new-round-fail / new-game-fail so generating does not block later actions
• Fix duplicate fixedPairs in localStorage cache serialization
• Apply court capacity when computing sit-outs (min(players, courts × 4))
• Cap generated matches to court count; fix lookahead simulation in getNextBestRound
• Add loading/disabled UI while a round is generating

Test plan

• yarn build passes (verified on DO dev VM)
• Manual: 7 players / 3 courts — round 2 renders, re-jumble and edit courts work
• yarn test:ci passes
• yarn lint passes

Closes #28

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