fix: wire up Add Employee screen

[jeffredodd]

Summary

Reworks the Add Employees screen reached from a Time-Off policy detail page so a single round-trip can manage the policy's roster end-to-end. Previously the user landed on a list of every non-terminated employee with no indication of who was already on the policy — re-submitting an existing assignee was easy and removing one was a separate flow on the detail page.

Two parts:

1. State machine wiring (already on this branch): viewTimeOffPolicyDetail now transitions to addEmployeesToPolicy on TIME_OFF_ADD_EMPLOYEES_TO_POLICY, reducing the payload's policyId into context. Round-trip back via the existing TIME_OFF_ADD_EMPLOYEES_DONE transition.
2. Manage-membership UX (new): the Add Employees screen now pre-selects existing assignees, shows their current balance read-only, diffs the selection at submit time, and gates removals behind a destructive confirm dialog.

Behavior details on (2):

• useSelectEmployeesData accepts initialSelectedUuids to seed the selection set.
• For originally-on-policy rows, the balance cell renders as read-only text (their current saved balance). Editable balance + carry-over pre-fill remains for newly-added rows.
• At submit, compute the diff against policy.employees. toAdd calls useTimeOffPoliciesAddEmployeesMutation; toRemove calls useTimeOffPoliciesRemoveEmployeesMutation. Unchanged rows are skipped — balances for existing assignees are owned by the dedicated edit-balance flow on the detail page.
• When toRemove.length > 0 a destructive confirm dialog gates the submit. On confirm, run remove first, then add, then invalidate the policy cache.
• Empty diff is a pure no-op: emits TIME_OFF_ADD_EMPLOYEES_DONE without an API call so navigation still works.
• Wizard mode is unchanged (no policy fetched, still emits employeeUuids).

This is PR 9 in the time-off bug-fix slate captured in ~/.claude/plans/ok-i-want-to-partitioned-balloon.md. Plan for the membership UX work: ~/.claude/plans/we-were-just-working-memoized-island.md.

Test plan

• npm run test -- --run src/components/UNSTABLE_TimeOff — 305/305 passing (added 6 new tests covering pre-select, read-only balance, no-op diff, add-only path, dialog-gated submit, remove-then-add ordering, cache invalidation)
• npx tsc --noEmit — clean
• npm run lint — clean
• Storybook: verify WithExistingAssignees, ExistingAssigneesWithNewSelections, and RemoveConfirmDialogOpen render correctly (npm run storybook)
• Manual e2e via SDK dev app (npm run sdk-app):
  • Open a Time-Off policy detail page that already has assignees → click "Add employees" → existing assignees appear pre-selected with their balance shown read-only
  • Uncheck one existing assignee, check one new employee → click Continue → confirm dialog appears → confirm → roster reflects both add and remove
  • Open the same screen and click Continue without changing anything → no API call, navigation returns to the detail page

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes a missing state-machine transition in the UNSTABLE Time Off flow so the “Add Employee” action from the policy detail screen actually navigates into the add-employees step and preserves the selected policyId in machine context.

Changes:

• Added a TIME_OFF_ADD_EMPLOYEES_TO_POLICY transition from viewTimeOffPolicyDetail → addEmployeesToPolicy, reducing policyId into context.
• Added unit tests covering the new transition and the round-trip back to viewTimeOffPolicyDetail via TIME_OFF_ADD_EMPLOYEES_DONE.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/components/UNSTABLE_TimeOff/TimeOffFlow/timeOffStateMachine.ts	Wires the previously-unhandled TIME_OFF_ADD_EMPLOYEES_TO_POLICY event to transition into addEmployeesToPolicy while updating context with policyId.
src/components/UNSTABLE_TimeOff/TimeOffFlow/timeOffStateMachine.test.ts	Adds regression tests validating the new transition and that policyId is preserved after returning from the add-employees step.

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[jeffredodd]

@copilot can you fix tests?

[Copilot]

@copilot can you fix tests?

Added the two missing state-machine transitions in viewTimeOffPolicyDetail (commit 8d2a2de):

• TIME_OFF_EDIT_POLICY → policyDetailsForm (sets policyId in context)
• TIME_OFF_CHANGE_SETTINGS → policySettings (sets policyId in context)

Also renamed the shared { policyId } payload type from AddEmployeesToPolicyPayload → PolicyIdPayload for clarity, and added two new unit tests covering both transitions. Suite is now 307/307 passing.