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/components/GridStackItem.java

@@ -4,10 +4,13 @@ import com.vaadin.flow.component.Component;
 import com.vaadin.flow.component.ComponentEvent;
 import com.vaadin.flow.component.ComponentEventListener;
 import com.vaadin.flow.component.contextmenu.MenuItem;
+import com.vaadin.flow.component.html.Anchor;
+import com.vaadin.flow.component.html.AttachmentType;
 import com.vaadin.flow.component.html.Div;
 import com.vaadin.flow.component.html.Span;
 import com.vaadin.flow.component.menubar.MenuBar;
 import com.vaadin.flow.component.menubar.MenuBarVariant;
+import com.vaadin.flow.server.streams.DownloadHandler;
 import com.vaadin.flow.shared.Registration;
 
 import java.util.Collections;
@@ -34,7 +37,9 @@ import java.util.UUID;
  * and {@link Action#REMOVE} are handled here, the rest is only reported to
  * {@link #addActionListener(ComponentEventListener) action listeners} — what
  * "refresh" or "duplicate" means depends on the widget, which this component
- * knows nothing about.
+ * knows nothing about. An entry that hands out a file rather than changing the
+ * widget is wired with
+ * {@link #setActionDownload(Action, DownloadHandler) setActionDownload}.
  */
 public class GridStackItem extends Div {
 
@@ -51,6 +56,12 @@ public class GridStackItem extends Div {
      *  button's. */
     public static final String ACTION_MENU_CLASS = "dialect-action-menu";
 
+    /** Marker class for the download link of a
+     *  {@link #setActionDownload(Action, DownloadHandler) download entry};
+     *  {@code styles.css} stretches it over the whole entry, so the whole row
+     *  is clickable and not just its caption. */
+    public static final String ACTION_LINK_CLASS = "dialect-action-link";
+
     /** Class set on the item itself while it is
      *  {@link #setMaximized(boolean) maximized} — {@code styles.css} is what
      *  actually lifts the item out of the grid. */
@@ -66,6 +77,10 @@ public class GridStackItem extends Div {
         MAXIMIZE,
         /** Add another widget of the same kind. Reported only. */
         DUPLICATE,
+        /** Download the data behind the widget. Reported only — the file itself
+         *  comes from the {@link #setActionDownload(Action, DownloadHandler)
+         *  download handler} the caller wired to this entry. */
+        EXPORT,
         /** Same as the close button, from the menu. Handled here. */
         REMOVE
     }
@@ -80,6 +95,7 @@ public class GridStackItem extends Div {
     private final MenuBar actionMenu = new MenuBar();
     private final Map<Action, MenuItem> actionItems = new EnumMap<>(Action.class);
     private final Map<Action, Span> actionCaptions = new EnumMap<>(Action.class);
+    private final Map<Action, Anchor> actionLinks = new EnumMap<>(Action.class);
     private boolean closable = true;
     private boolean maximized;
 
@@ -151,6 +167,7 @@ public class GridStackItem extends Div {
         addActionItem(root, Action.REFRESH, Fa.REFRESH, "gridstack.refresh");
         addActionItem(root, Action.MAXIMIZE, Fa.MAXIMIZE, "gridstack.maximize");
         addActionItem(root, Action.DUPLICATE, Fa.DUPLICATE, "gridstack.duplicate");
+        addActionItem(root, Action.EXPORT, Fa.EXPORT, "gridstack.export");
         addActionItem(root, Action.REMOVE, Fa.REMOVE, "gridstack.remove");
 
         setActions(DEFAULT_ACTIONS);
@@ -189,6 +206,36 @@ public class GridStackItem extends Div {
         return this;
     }
 
+    /**
+     * Makes an entry hand out a file: its caption becomes a download link over
+     * {@code handler}, so picking it downloads instead of only firing an
+     * {@link ActionEvent} (which it still does — a listener can react to the
+     * export as well).
+     * <p>
+     * The handler is asked for its content when the entry is clicked, not here,
+     * so a widget that has been re-fed in the meantime exports what it is
+     * showing at that moment. Calling this again re-points the same link.
+     */
+    public GridStackItem setActionDownload(Action action, DownloadHandler handler) {
+        Anchor link = actionLinks.computeIfAbsent(action, key -> {
+            Anchor anchor = new Anchor();
+            anchor.addClassName(ACTION_LINK_CLASS);
+            // Reparents the caption into the link: the entry keeps its icon and
+            // its text, only the text is now what the browser downloads from.
+            anchor.add(actionCaptions.get(key));
+            actionItems.get(key).add(anchor);
+            return anchor;
+        });
+        link.setHref(handler, AttachmentType.DOWNLOAD);
+        return this;
+    }
+
+    /** The download link of an entry wired with
+     *  {@link #setActionDownload(Action, DownloadHandler)}, if it has one. */
+    public Anchor getActionLink(Action action) {
+        return actionLinks.get(action);
+    }
+
     /** Shows or hides a single menu entry — the widget types that support an
      *  action differ, the menu does not. */
     public GridStackItem setActionEnabled(Action action, boolean enabled) {