feat v1.33.0: background editor — visual decor layer

- third mode tab: draw lines/rects/ovals/text with grid snap, drag
  preview; select+move (Delete key), erase, color/width/fill controls;
  Esc ladder; text dialog with S/M/L sizes and dblclick re-edit
- decor renders under rooms, visible in every mode, click-transparent
  outside its editor; stored in space.decor with backend schema
  (line/rect/ellipse/text variants), rev/optimistic locking as usual
- smoke_decor.mjs (19 checks); smoke_editor_tabs updated to 3 tabs;
  TESTING/CHANGELOG/UX-MODES same-commit

demo/smoke_editor_tabs.mjs

@@ -6,7 +6,7 @@ const res = await page.evaluate(async () => {
   const sr = () => c.shadowRoot || c.renderRoot;
   const tabs = () => [...sr().querySelectorAll('.modetab')];
   // 1) две вкладки, Просмотра нет, крестиков в неактивных нет
-  out.twoTabs = tabs().length === 2;
+  out.twoTabs = tabs().length === 3; // третья — Редактор подложки (v1.33.0)
   out.labels = tabs().map((t) => t.textContent.trim());
   out.noCrossIdle = sr().querySelectorAll('.modetab .closex').length === 0;
   out.startView = c._mode === 'view';