fix: add a per-widget action menu (#27)

A GridStackItem now carries an overflow menu next to the grip and the
close button: refresh, maximize, duplicate, remove. Maximize and remove
are handled by the item itself; refresh and duplicate are only reported,
since what they mean depends on the widget.

Maximizing only sets a class — the item's gs-* attributes and its
gridstack node are untouched, so restoring is by definition the position
it had.

The menu sits outside the drag-handle selector, like the close button, so
opening it never starts a drag; a Playwright test drags it to prove it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0124BiJikbhbiEsNfJxdWM69

src/test/java/com/example/widgets/WidgetRegistryTest.java

@@ -1,10 +1,12 @@
 package com.example.widgets;
 
 import com.example.data.InMemoryChartDataService;
+import com.vaadin.flow.component.html.Div;
 import org.junit.jupiter.api.Test;
 
 import java.util.List;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
@@ -29,6 +31,14 @@ class WidgetRegistryTest {
                 "every definition carries a usable default size");
     }
 
+    /** A widget with no refresher must not blow up when one is asked for: the
+     *  dashboard offers the action per widget, the registry only carries it
+     *  out. */
+    @Test
+    void refresh_ignoresAWidgetThatCarriesNoRefresher() {
+        assertDoesNotThrow(() -> WidgetRegistry.refresh(new Div()));
+    }
+
     @Test
     void require_rejectsAnUnknownType() {
         assertThrows(IllegalArgumentException.class, () -> registry.require("nope"));