Sit-out modal — draft pre-gen + correct volunteer state

[javaisbetterthanpython]

Summary

Upstream: pickleball-shuffler#32

Fix volunteer sit-out modal stale state and integrate with background pre-generation. Volunteers should reflect the current round's saved state when opening — not leftovers from a previous edit session.

Flow

Open modal

• Pre-check volunteers from current round's saved state.

Edit (draft only — no live save)

• Checkbox changes are draft until Re-jumble is clicked.
• Each draft change triggers debounced (150ms) background pre-generation for the current round.
• Pre-gen uses draft volunteer list (not saved until confirmed).

Re-jumble

• Applies draft volunteer selection (saves state).
• Instantly shows pre-computed round layout (no perceptible wait).

Cancel / close without Re-jumble

• Discard draft pre-generation.
• Saved volunteer state and displayed round unchanged.

Acceptance criteria

• Modal opens with current round's saved volunteers pre-selected
• Checkbox edits are draft-only (not saved until Re-jumble)
• 150ms debounced background pre-gen on draft changes
• Re-jumble saves draft + displays pre-computed layout instantly
• Cancel/close discards draft pre-gen; no state change
• No stale volunteer selections from prior modal sessions
• Coordinates with Richer session mix + instant next round #30 pre-gen infrastructure
• yarn test:ci and yarn build pass

Files likely touched

• src/SitoutsModal.tsx
• pages/rounds.tsx
• src/useShuffler.tsx

Dependencies

• Richer session mix + instant next round #30 (shared background pre-generation infrastructure; can implement in parallel once Richer session mix + instant next round #30 lands)

Upstream reference

• Volunteer sit-out modal doesn't clear between openings/rounds panathea/pickleball-shuffler#32