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_en.properties

@@ -31,14 +31,24 @@ gridstack.pickerTitle=Choose a widget
 gridstack.dragHandle=Move
 gridstack.close=Close
 
+filter.period=Period
+filter.period.month=Month
+filter.period.quarter=Quarter
+filter.period.halfYear=Half year
+filter.period.year=Year
+filter.region=Region
+filter.allRegions=All regions
+
+# The KPI values are MessageFormat patterns: the number comes from the data
+# service (it depends on the filter), unit and number format from the bundle.
 kpi.revenueTotal=Total revenue
-kpi.revenueTotal.value=€1.24M
+kpi.revenueTotal.value=€{0,number,#,##0.00}M
 kpi.openOrders=Open orders
-kpi.openOrders.value=38
+kpi.openOrders.value={0,number,#,##0}
 kpi.newCustomers=New customers
-kpi.newCustomers.value=112
+kpi.newCustomers.value={0,number,#,##0}
 kpi.averageOrderValue=Avg. order value
-kpi.averageOrderValue.value=€486
+kpi.averageOrderValue.value=€{0,number,#,##0}
 
 chart.revenueSeries=Revenue 2026
 chart.pointClick=Series {0}, point {1}
@@ -50,6 +60,17 @@ month.mar=Mar
 month.apr=Apr
 month.may=May
 month.jun=Jun
+month.jul=Jul
+month.aug=Aug
+month.sep=Sep
+month.oct=Oct
+month.nov=Nov
+month.dec=Dec
+
+week.1=Week 1
+week.2=Week 2
+week.3=Week 3
+week.4=Week 4
 
 region.north=North
 region.south=South