Harden command zone tax counter handling and more clean up

- Initialize commandZoneGraphicsItem to nullptr so the pervasive null
    checks have defined behavior before initializeZones() runs
  - Remove unused getTaxCounterWidgets()
  - Enforce the "reset tax to 0 before deactivating" invariant server-side
    in cmdSetCounterActive, not just in the UI
  - Hide tax increment/decrement menu actions at their bounds (0 and
    MAX_COUNTER_VALUE) to avoid sending no-op commands
  - Drop redundant/inconsistent toggle labels from CommandZoneMenu::
    retranslateUi(); updateTaxCounterActionStates() owns them
  - Add default=-1 to Event_SetCounterActive.counter_id for parity with
    Command_SetCounterActive

libcockatrice_protocol/libcockatrice/protocol/pb/event_set_counter_active.proto

@@ -5,6 +5,6 @@ message Event_SetCounterActive {
     extend GameEvent {
         optional Event_SetCounterActive ext = 2023;
     }
-    optional sint32 counter_id = 1;
+    optional sint32 counter_id = 1 [default = -1];
     optional bool active = 2 [default = true];
 }