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/web/src/ShiftsHistory.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useState } from "react";
 import { useTranslation } from "react-i18next";
-import { useQuery } from "@tanstack/react-query";
+import { keepPreviousData, useQuery } from "@tanstack/react-query";
 import {
   closeShift,
   fetchEvents,
@@ -105,9 +105,17 @@ export function ShiftsHistory({ user, canManage = false }: { user: SessionUser |
     to: range?.to ? new Date(`${range.to}T23:59:59`).toISOString() : undefined,
   };
 
-  const q = useQuery({ queryKey: ["shifts", applied], queryFn: () => fetchShifts(applied) });
+  // keepPreviousData: every filter change makes a NEW query key; without it the
+  // data (and with it `scope`) goes undefined for the fetch round-trip, which
+  // unmounted the admin filter controls mid-interaction and blanked the list.
+  const q = useQuery({
+    queryKey: ["shifts", applied],
+    queryFn: () => fetchShifts(applied),
+    placeholderData: keepPreviousData,
+  });
   const isAdmin = q.data?.scope === "all";
   const closed = q.data?.shifts ?? [];
+  const operators = q.data?.operators ?? [];
 
   // The current/open shift sits at the TOP of the list (when present + visible to me).
   const list: (ShiftSummary & { open?: boolean })[] = current && (isMine || isAdmin) ? [current, ...closed] : closed;
@@ -169,7 +177,14 @@ export function ShiftsHistory({ user, canManage = false }: { user: SessionUser |
         {isAdmin && (
           <div className="field">
             <span className="label">{t("shifts.operator")}</span>
-            <input className="input w-44" value={operator} onChange={(e) => setOperator(e.target.value)} placeholder={t("shifts.allOperators")} />
+            {/* A select over operators that HAVE shifts — the server filter is an
+                exact username match, so free text could only miss. */}
+            <select className="input w-44" value={operator} onChange={(e) => setOperator(e.target.value)}>
+              <option value="">{t("shifts.allOperators")}</option>
+              {operators.map((op) => (
+                <option key={op} value={op}>{op}</option>
+              ))}
+            </select>
           </div>
         )}
       </div>