Patient page: assign a gap inline, without going back to a list

[Taleef7]

What

On the patient page (/employees/[externalId]), list every open gap with an inline assign select per gap, so a case can be assigned without leaving the page.

Named in the MM-2 plan under PR 1 and deliberately not built there — #551 was already large, and this is additive rather than something the rest depends on.

Why it is worth doing

The patient page is where someone lands after clicking a name on /worklist, and it is currently read-only for assignment: to assign one of the patient's gaps you go back to a list, find the row, tick it, and use the bulk control. The work list exists to let one phone call close what one call can close; the patient page is where that call actually happens.

What already exists

Everything needed is in place after #551 — this is wiring, not new mechanism:

• useAssignableUsers (frontend/features/panel/use-assignable-users.ts) — the options, the canonical-spelling resolver, and the UNASSIGN_VALUE sentinel
• POST /api/cases/:id/assign — server-side validated against the same list the hook offers
• EmployeeProfileResponse.openCases already carries the gaps

Notes for whoever picks it up

• The select must show an assignee no longer in the assignable set as such, rather than rendering blank over a case that is in fact assigned — the trap fix(case): an assignee is chosen from the accounts that exist, and a measure's roster is the patients it describes #550 hit. canonicalFor answers that question; use it for both the truth test and the displayed value.
• Disable the control when the choice would write what is already stored, which also avoids a no-op audit event.
• If more than one gap is assigned at once, prefer POST /api/cases/bulk-assign over a loop — it is one audited statement, and the per-case loop is what feat(worklist): a patient-first work list, panel filters by PCP and insurance, and set-based bulk assign #551 removed from /cases.

Context: #551, MM-2 plan PR 1(B).