fix: export widget data as CSV from the action menu (#28)

The ApexCharts toolbar stays hidden (DialectTheme), so the dashboard had no
export at all. Adds an "Als CSV exportieren" entry to the per-widget action
menu, served server-side from the same data the widget renders.

- CsvExport: the export table plus its CSV dialect (';' separator, CRLF,
  UTF-8 BOM, locale-formatted numbers) and file-name slugging.
- GridStackItem.setActionDownload: turns a menu entry's caption into an
  anchor over a DownloadHandler, so an entry can hand out a file.
- WidgetRegistry: keeps each widget's query, not only its result, so the
  Export hook re-runs it — an export always matches the current filter.
- DashboardView: builds the download on click; file name is widget title +
  period (revenue-trend-half-year.csv).

Chart image export is left as the follow-up the issue calls optional.

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

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

@@ -9,6 +9,7 @@ import com.example.components.KpiTile;
 import com.example.data.ChartDataService;
 import com.example.data.DashboardFilter;
 import com.example.data.KpiData;
+import com.example.export.CsvExport;
 import com.example.widgets.DashboardContext;
 import com.example.widgets.WidgetDefinition;
 import com.example.widgets.WidgetRegistry;
@@ -22,10 +23,17 @@ import com.vaadin.flow.component.orderedlayout.HorizontalLayout;
 import com.vaadin.flow.component.orderedlayout.VerticalLayout;
 import com.vaadin.flow.router.HasDynamicTitle;
 import com.vaadin.flow.router.Route;
+import com.vaadin.flow.server.HttpStatusCode;
+import com.vaadin.flow.server.VaadinSession;
+import com.vaadin.flow.server.streams.DownloadHandler;
+import com.vaadin.flow.server.streams.DownloadResponse;
 
+import java.io.ByteArrayInputStream;
 import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
+import java.util.Optional;
 
 /**
  * The dashboard: a {@link GridStackLayout} of draggable/resizable cards whose
@@ -172,7 +180,9 @@ public class DashboardView extends VerticalLayout implements HasDynamicTitle {
         Component content = definition.factory().apply(context);
         GridStackItem item = new GridStackItem(id, x, y, w, h,
                 new Card(getTranslation(definition.titleKey()), content));
-        item.setActions(Action.REFRESH, Action.MAXIMIZE, Action.DUPLICATE, Action.REMOVE);
+        item.setActions(Action.REFRESH, Action.MAXIMIZE, Action.DUPLICATE,
+                Action.EXPORT, Action.REMOVE);
+        item.setActionDownload(Action.EXPORT, csvDownload(definition, content));
         item.addActionListener(e -> {
             switch (e.getAction()) {
                 case REFRESH -> WidgetRegistry.refresh(content);
@@ -183,6 +193,41 @@ public class DashboardView extends VerticalLayout implements HasDynamicTitle {
         return item;
     }
 
+    /**
+     * The widget's data as a CSV attachment. Nothing is computed here: the
+     * callback runs when the user picks the entry, so data, labels and file
+     * name are all of the moment — including whatever the filter bar is set to
+     * then.
+     * <p>
+     * A download is served on a request of its own, outside the session lock
+     * and without a current {@code UI} (see
+     * {@code StreamRequestHandler#callElementResourceHandler}), so the lock is
+     * taken for the read and the locale is passed explicitly.
+     */
+    private DownloadHandler csvDownload(WidgetDefinition definition, Component content) {
+        return DownloadHandler.fromInputStream(event -> {
+            VaadinSession session = event.getSession();
+            session.lock();
+            Locale locale;
+            String fileName;
+            Optional<CsvExport> table;
+            try {
+                locale = event.getUI().getLocale();
+                table = WidgetRegistry.export(content, locale);
+                fileName = CsvExport.fileName(getTranslation(locale, definition.titleKey()),
+                        getTranslation(locale, context.getFilter().period().labelKey()));
+            } finally {
+                session.unlock();
+            }
+            if (table.isEmpty()) {
+                return DownloadResponse.error(HttpStatusCode.NOT_FOUND);
+            }
+            byte[] csv = table.get().toBytes(locale);
+            return new DownloadResponse(new ByteArrayInputStream(csv), fileName,
+                    "text/csv;charset=utf-8", csv.length);
+        });
+    }
+
     /** Re-feeds a single tile from the current filter — the action menu's
      *  refresh, which asks for one widget, not for the dashboard. */
     private void refreshKpiTile(String kpiId) {