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/Fa.java

@@ -31,6 +31,7 @@ public enum Fa {
     REFRESH("fa-solid", "fa-arrows-rotate"),
     MAXIMIZE("fa-solid", "fa-expand"),
     DUPLICATE("fa-solid", "fa-clone"),
+    EXPORT("fa-solid", "fa-file-csv"),
     TREND_UP("fa-solid", "fa-arrow-trend-up"),
     TREND_DOWN("fa-solid", "fa-arrow-trend-down");