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/java/com/example/widgets/DashboardContext.java

@@ -0,0 +1,63 @@
+package com.example.widgets;
+
+import com.example.data.ChartDataService;
+import com.example.data.DashboardFilter;
+import com.vaadin.flow.shared.Registration;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+import java.util.function.Consumer;
+
+/**
+ * The dashboard's current {@link DashboardFilter} plus the widgets listening to
+ * it — the small event bus between the filter bar (which sets) and the widgets
+ * (which subscribe and re-request their data from
+ * {@link ChartDataService}). Neither side knows the other.
+ * <p>
+ * One context per dashboard instance, deliberately not a Spring bean: it is
+ * view state, so nothing leaks between users, tabs or visits. For the same
+ * reason the filter is <em>not persisted</em> — a reload starts from
+ * {@link DashboardFilter#defaults()}, unlike the grid layout, which
+ * {@code GridStackLayout} keeps in {@code localStorage}.
+ */
+public class DashboardContext {
+
+    private final List<Consumer<DashboardFilter>> listeners = new ArrayList<>();
+    private DashboardFilter filter = DashboardFilter.defaults();
+
+    public DashboardFilter getFilter() {
+        return filter;
+    }
+
+    /**
+     * Applies a filter and hands it to every listener. A filter equal to the
+     * current one changes nothing and notifies nobody, so re-picking the same
+     * value in the filter bar does not make the widgets re-fetch.
+     */
+    public void setFilter(DashboardFilter filter) {
+        Objects.requireNonNull(filter, "filter");
+        if (this.filter.equals(filter)) {
+            return;
+        }
+        this.filter = filter;
+        // Over a copy: a listener may unsubscribe while being notified (a
+        // widget closing in reaction to the update).
+        List.copyOf(listeners).forEach(listener -> listener.accept(filter));
+    }
+
+    /**
+     * Subscribes to later filter changes. The listener is not called with the
+     * current filter — a widget is built from it already — so a subscriber
+     * feeds itself once and then only reacts.
+     */
+    public Registration addFilterChangeListener(Consumer<DashboardFilter> listener) {
+        listeners.add(listener);
+        return () -> listeners.remove(listener);
+    }
+
+    /** How many widgets are currently listening; for tests and diagnostics. */
+    public int getListenerCount() {
+        return listeners.size();
+    }
+}