fix(editor): shrink NPC markers, add show/hide toggle

- NPC markers reduced from 30-unit poles to 8-unit poles (was
  overwhelming and obscuring terrain). Base 1.5u, diamond top 1u
- "Show Position Markers" checkbox in NPC panel to toggle visibility
- Markers hidden when checkbox unchecked — useful when M2 creature
  models are rendering and markers are redundant
- Marker alpha reduced for less visual noise

tools/editor/editor_ui.cpp

@@ -1748,6 +1748,11 @@ void EditorUI::renderNpcPanel(EditorApp& app) {
         auto& presets = app.getNpcPresets();
         auto& tmpl = spawner.getTemplate();
 
+        bool showMarkers = app.getViewport().getShowNpcMarkers();
+        if (ImGui::Checkbox("Show Position Markers", &showMarkers))
+            app.getViewport().setShowNpcMarkers(showMarkers);
+        ImGui::Separator();
+
         // ---- Creature Browser ----
         if (ImGui::CollapsingHeader("Creature Browser", ImGuiTreeNodeFlags_DefaultOpen)) {
             // Category filter