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/test/java/com/example/widgets/WidgetRegistryTest.java

@@ -38,7 +38,7 @@ class WidgetRegistryTest {
     void register_replacesTheDefinitionOfTheSameType() {
         int before = registry.definitions().size();
         WidgetDefinition replacement = new WidgetDefinition(WidgetRegistry.REVENUE_TREND,
-                "card.revenueTrend", 2, 2, () -> null);
+                "card.revenueTrend", 2, 2, context -> null);
 
         registry.register(replacement);