fix(shifts): operator filter — select over real operators, no more focus loss

The admin operator filter was a free-text input that broke three ways at
once: its visibility hangs off the query response (scope === "all") and
its value is part of the query key, so every keystroke started a new
query, data went undefined for the round-trip, and the input UNMOUNTED
mid-keystroke (lost focus, list blanking that read as a page reload).
Filtering also silently failed — the server matches the operator by
exact username, so partial text matched nothing.

- keepPreviousData on the shifts query: previous data (and scope) stays
  live during refetch, so filter controls never unmount and the list
  never blanks on preset/filter changes.
- The filter is now a <select> of operators that HAVE shifts: the server
  returns the distinct list (signed z-reports + the open shift's holder)
  on GET /api/shifts, admin scope only — operators still can't see other
  names. Exact match by construction.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

apps/server/src/routes/shift.ts

@@ -63,7 +63,10 @@ export async function shiftRoutes(app: FastifyInstance, shift: ShiftService): Pr
     const from = canSeeAll ? q.from?.trim() || undefined : undefined;
     const to = canSeeAll ? q.to?.trim() || undefined : undefined;
     const shifts = shift.listShifts({ operator, from, to });
-    return { shifts, scope: canSeeAll ? "all" : "self" };
+    // Admins also get the distinct operator list (unfiltered) for the filter
+    // dropdown — operators don't see other names, so it's scope-gated.
+    if (canSeeAll) return { shifts, scope: "all", operators: shift.listOperators() };
+    return { shifts, scope: "self" };
   });
 
   // NB: drawer cash movements (record/review) moved to routes/drawer.ts (2026-07-01) — the