fix: address code review findings from crop/text commit

- CropOverlay: register move/up handlers dynamically on drag start (single
  handler instead of 8x per-handle), remove on drag end. Override destroy()
  to call _cleanup() preventing keyboard listener leaks.
- Text format toolbar: re-measure text bounds (w/h) after fontSize/fontFamily
  changes, update spatial index and transform box.
- TextEditor: add clearText() method; tools.ts uses it instead of fragile
  document.querySelector('textarea').
- SceneManager._updateItem: apply crop mask on remote sync for image items.
- useCanvasSetup: stop TextEditor on unmount to prevent orphaned textarea.
- Double-click zoom: use item.data dimensions instead of getBounds() (which
  includes shadow offset).

frontend/src/canvas/tools.ts

@@ -130,8 +130,7 @@ export function activateTool(
               ctx.onChange();
             });
             // Clear placeholder so user types from scratch
-            const ta = document.querySelector('textarea');
-            if (ta) { ta.value = ''; }
+            ctx.textEditor!.clearText();
           }, 50);
         }