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/shift-service.test.ts

@@ -234,4 +234,11 @@ describe("close signs a Z-report; listShifts reads it back", () => {
     await shift.open("bob"); await shift.close("bob");
     expect(shift.listShifts({ operator: "alice" }).map((s) => s.operator)).toEqual(["alice"]);
   });
+
+  it("listOperators: distinct + sorted, includes the OPEN shift's operator", async () => {
+    await shift.open("bob"); await shift.close("bob");
+    await shift.open("bob"); await shift.close("bob"); // twice — must stay distinct
+    await shift.open("alice"); // open, no z-report yet
+    expect(shift.listOperators()).toEqual(["alice", "bob"]);
+  });
 });