fix: merge GridStackView into DashboardView (#19)

DashboardView and GridStackView rendered the same three charts from the
same hardcoded data, so every chart factory method existed twice. The
dashboard is now the GridStackLayout variant at @Route(""):

- GridStackView deleted, its grid/toolbar/chart factories moved into
  DashboardView, which keeps the point-click notifications the old
  dashboard had.
- Charts size to their widget (100%/100%) instead of a fixed 400px.
- MainLayout has one dashboard entry; nav.gridstack/page.gridstack keys
  dropped from all three bundles.
- Storage key is "dashboard" (was "gridstack-demo"), so a saved layout
  from the old route is not reused.
- GridStackViewTest renamed to DashboardViewTest, plus a test asserting
  the three charts render.

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

src/main/java/com/example/views/MainLayout.java

@@ -49,8 +49,6 @@ public class MainLayout extends AppLayout {
                 Fa.FORM.create()));
         nav.addItem(new SideNavItem(getTranslation("nav.table"), TableView.class,
                 Fa.TABLE.create()));
-        nav.addItem(new SideNavItem(getTranslation("nav.gridstack"), GridStackView.class,
-                Fa.GRID.create()));
 
         addToDrawer(nav);
     }