fix: undo closing a widget (#40)

Closing a widget was instant and destructive - one misclick on the X
lost its size and position. Close now shows a toast with an undo
button that re-adds the very same widget at its captured gs-x/y/w/h,
keeping its gs-id. Programmatic close() (fromClient=false) is skipped.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HGRChzremwYCUctf2qMrQz

src/main/java/com/example/components/GridStackItem.java

@@ -312,10 +312,26 @@ public class GridStackItem extends Div {
     }
 
     private void close(boolean fromClient) {
+        // Captured before detaching: once removeFromParent() runs, the grid no
+        // longer reports this item, so its position could not be read back from
+        // there afterwards. The gs-* attributes themselves are untouched by the
+        // detach, so a caller that keeps this item around and re-adds it later
+        // (e.g. to undo the close) lands it back at exactly this spot.
+        Position position = capturePosition();
         // Detach first so listeners observe the layout they are about to see —
         // getLayout() on the grid no longer counts this item.
         getElement().removeFromParent();
-        fireEvent(new CloseEvent(this, fromClient));
+        fireEvent(new CloseEvent(this, fromClient, position));
+    }
+
+    private Position capturePosition() {
+        return new Position(getItemId(),
+                attrInt("gs-x"), attrInt("gs-y"), attrInt("gs-w"), attrInt("gs-h"));
+    }
+
+    private int attrInt(String name) {
+        String value = getElement().getAttribute(name);
+        return value == null ? 0 : Integer.parseInt(value);
     }
 
     /** Shows or hides the close button (shown by default). */
@@ -346,10 +362,23 @@ public class GridStackItem extends Div {
         return addListener(CloseEvent.class, listener);
     }
 
-    /** Fired after the item has been removed from its {@link GridStackLayout}. */
+    /** Fired after the item has been removed from its {@link GridStackLayout}.
+     *  Carries the {@link Position} it held right before detaching, so a
+     *  listener can offer to undo the close (re-adding the item at that exact
+     *  spot) without having to track positions itself. {@link #isFromClient()}
+     *  tells apart a user-initiated close (button/menu) from a programmatic
+     *  {@link GridStackItem#close()} — an undo affordance only makes sense for
+     *  the former. */
     public static class CloseEvent extends ComponentEvent<GridStackItem> {
-        CloseEvent(GridStackItem source, boolean fromClient) {
+        private final Position position;
+
+        CloseEvent(GridStackItem source, boolean fromClient, Position position) {
             super(source, fromClient);
+            this.position = position;
+        }
+
+        public Position getPosition() {
+            return position;
         }
     }