[#33] Skill groups — foundation (data model + UI)

[javaisbetterthanpython]

Summary

• Adds groups.ts data model (Standard/Advanced defaults, swing detection, fixed-pair leveling) with localStorage via useShuffler.
• Adds GroupsEditor / GroupsModal for enable toggle, drag priority, player assignment, and swing warnings on /new, Players modal, and rounds.
• Records per-round roundPlayModes and nextRoundPlayMode override for upcoming Skill groups — court-to-group mapping + rounds layout #34 generation work.

Test plan

• npm run test:ci (including 7 new groups.spec.ts tests)
• npm run build
• npm run lint
• Manual: enable skill groups on /new, assign players, start game
• Manual: open Groups on rounds, change play mode for next round
• Manual: verify swing ⚠️ on multi-group players

Closes #33

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