fix: restore saved gridstack layout faithfully (#13)

The layout was saved to localStorage, but restoring it did not reliably
reproduce it:

- restore() applied the saved nodes one by one via grid.update(). A later
  item could collide with one already put back and push it off its saved
  spot, and nothing moved it back — so the restored layout was not the one
  the user left. Use grid.load(nodes, false) instead: it sorts the nodes,
  removes them from the engine before re-placing them, and runs in a single
  batch. addRemove is off because Flow owns which children exist.
- gridstack's save() omits w/h when they are 1. The manual restore passed
  them through as undefined, and the server read them as 0 and wrote a
  zero-sized widget back onto the item. load() re-applies gridstack's own
  defaults; onLayoutChange now reads a missing w/h as 1.
- A drag followed immediately by navigating to another route detached the
  element inside the 150ms persist debounce, dropping the pending save.
  disconnectedCallback now flushes it to localStorage first.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ue9ZtWUBQF4SuSHpzZ3zwq

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

@@ -179,18 +179,17 @@ public class GridStackLayout extends Component implements HasSize, HasStyle {
             String id = node.path("id").asString(null);
             if (id == null) continue;
 
-            findItem(id).ifPresent(item -> item.setPosition(
-                    node.path("x").asInt(),
-                    node.path("y").asInt(),
-                    node.path("w").asInt(),
-                    node.path("h").asInt()));
+            // gridstack omits w/h from its saved layout when they are 1, so a
+            // missing value means 1 — defaulting to 0 wrote a zero-sized widget
+            // back onto the item and lost its size on the next attach.
+            int x = node.path("x").asInt(0);
+            int y = node.path("y").asInt(0);
+            int w = node.path("w").asInt(1);
+            int h = node.path("h").asInt(1);
 
-            positions.add(new GridStackItem.Position(
-                    id,
-                    node.path("x").asInt(),
-                    node.path("y").asInt(),
-                    node.path("w").asInt(),
-                    node.path("h").asInt()));
+            findItem(id).ifPresent(item -> item.setPosition(x, y, w, h));
+
+            positions.add(new GridStackItem.Position(id, x, y, w, h));
         }
 
         fireEvent(new LayoutChangeEvent(this, true, positions));