fix: add a global dashboard filter bar (#24)

The reporting period was baked into the data service, so nothing could
change what the dashboard shows without editing code.

- DashboardFilter (period + optional region) parameterises every
  ChartDataService query; the no-arg overloads are the default filter.
- DashboardContext is the bus between the new DashboardFilterBar and the
  widgets: charts subscribe in WidgetRegistry, KPI tiles in DashboardView.
- Updates go through updateData, so widgets patch in place instead of
  being rebuilt; SparklineChart gained the same path.
- KpiData carries the number, the bundle the unit and number pattern, so
  a KPI value can follow the filter and stay locale-formatted.
- Filter state is deliberately not persisted, unlike the grid layout.

Closes #24

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

src/main/resources/vaadin-i18n/translations.properties

@@ -31,14 +31,24 @@ gridstack.pickerTitle=Widget auswählen
 gridstack.dragHandle=Verschieben
 gridstack.close=Schließen
 
+filter.period=Zeitraum
+filter.period.month=Monat
+filter.period.quarter=Quartal
+filter.period.halfYear=Halbjahr
+filter.period.year=Jahr
+filter.region=Region
+filter.allRegions=Alle Regionen
+
+# Die KPI-Werte sind MessageFormat-Muster: die Zahl kommt aus dem Datenservice
+# (sie hängt vom Filter ab), Einheit und Zahlenformat aus dem Bundle.
 kpi.revenueTotal=Gesamtumsatz
-kpi.revenueTotal.value=1,24 Mio. €
+kpi.revenueTotal.value={0,number,#,##0.00} Mio. €
 kpi.openOrders=Offene Vorgänge
-kpi.openOrders.value=38
+kpi.openOrders.value={0,number,#,##0}
 kpi.newCustomers=Neue Kunden
-kpi.newCustomers.value=112
+kpi.newCustomers.value={0,number,#,##0}
 kpi.averageOrderValue=Ø Bestellwert
-kpi.averageOrderValue.value=486 €
+kpi.averageOrderValue.value={0,number,#,##0} €
 
 chart.revenueSeries=Umsatz 2026
 chart.pointClick=Serie {0}, Punkt {1}
@@ -50,6 +60,17 @@ month.mar=Mär
 month.apr=Apr
 month.may=Mai
 month.jun=Jun
+month.jul=Jul
+month.aug=Aug
+month.sep=Sep
+month.oct=Okt
+month.nov=Nov
+month.dec=Dez
+
+week.1=Woche 1
+week.2=Woche 2
+week.3=Woche 3
+week.4=Woche 4
 
 region.north=Nord
 region.south=Süd