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/api.ts

@@ -1151,6 +1151,8 @@ export interface ShiftSummary extends ShiftSourceSplit {
 export function fetchShifts(params: { operator?: string; from?: string; to?: string } = {}): Promise<{
   shifts: ShiftSummary[];
   scope: "all" | "self";
+  /** Admin scope only: every operator that has a shift — feeds the filter dropdown. */
+  operators?: string[];
 }> {
   const qs = new URLSearchParams();
   if (params.operator) qs.set("operator", params.operator);