fix(charts): paint empty-window state instead of erasing the card

`health-chart.tsx` returned `null` when the loaded payload was
empty in the selected range, which stripped the entire widget
(header + tabs + chart). On the dashboard the user saw a missing
tile with no explanation. Keep the card chrome mounted and paint a
`<ChartEmptyState>` with distinct copy from the < 3-points
"Mehr Messtage erforderlich" branch so the two situations don't
blur. Adds `charts.noDataInRange*` strings across all six locales.

src/components/charts/__tests__/health-chart-no-data-in-range.test.tsx

@@ -0,0 +1,77 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { renderToStaticMarkup } from "react-dom/server";
+
+/**
+ * v1.4.43 W11-M6 — chart empty-window state.
+ *
+ * Pre-fix the chart returned `null` when the loaded data array was
+ * empty in the selected range, erasing the entire card (header +
+ * tabs + chart). The user saw a missing widget without explanation.
+ * The fix paints a "no data in this range" empty state via
+ * `<ChartEmptyState>` so the dashboard composition stays intact and
+ * the user knows the chart loaded but the selected window holds no
+ * measurements. The copy is distinct from the < 3-points "Mehr
+ * Messtage erforderlich" branch so the two situations don't blur.
+ */
+
+function buildData(rawCount: number): unknown[] {
+  const points: unknown[] = [];
+  Object.defineProperty(points, "rawCount", {
+    value: rawCount,
+    enumerable: false,
+    configurable: false,
+    writable: false,
+  });
+  return points;
+}
+
+vi.mock("@/hooks/use-auth", () => ({
+  useAuth: () => ({
+    isAuthenticated: true,
+    user: null,
+    isLoading: false,
+  }),
+}));
+
+describe("<HealthChart> — empty-window state (W11-M6)", () => {
+  beforeEach(() => {
+    vi.resetModules();
+  });
+
+  it("renders the no-data-in-range copy when data resolves to an empty array", async () => {
+    const data = buildData(0);
+
+    vi.doMock("@tanstack/react-query", () => ({
+      useQuery: () => ({ data, isLoading: false }),
+      useQueryClient: () => ({
+        cancelQueries: () => Promise.resolve(),
+        getQueryData: () => undefined,
+        setQueryData: () => undefined,
+        invalidateQueries: () => Promise.resolve(),
+      }),
+      useMutation: () => ({ mutate: () => undefined, isPending: false }),
+    }));
+
+    const { I18nProvider } = await import("@/lib/i18n/context");
+    const { HealthChart } = await import("../health-chart");
+
+    const html = renderToStaticMarkup(
+      <I18nProvider initialLocale="de">
+        <HealthChart types={["PULSE"]} title="Pulse" unit="bpm" />
+      </I18nProvider>,
+    );
+
+    // The new empty state must paint…
+    expect(html).toContain("Keine Daten in diesem Zeitraum");
+    // …and the chart card chrome (title) must still be mounted so the
+    // dashboard isn't missing a tile. The pre-fix `return null` would
+    // have stripped this entirely.
+    expect(html).toContain("Pulse");
+    // The < 3-points branch must NOT paint here — empty-window and
+    // sparse-data are different situations with distinct copy.
+    expect(html).not.toContain("Mehr Messtage erforderlich");
+    expect(html).not.toContain("Erfasse mindestens 3 Einträge");
+
+    vi.doUnmock("@tanstack/react-query");
+  });
+});

src/components/charts/health-chart.tsx

@@ -1132,7 +1132,14 @@ export function HealthChart({
     return map;
   }, [chartData, types]);
 
-  if (!isLoading && !data?.length) return null;
+  // v1.4.43 W11-M6 — keep the card shell mounted even when the loaded
+  // payload is empty. Pre-fix the early `return null` here erased the
+  // whole widget (header + range tabs + chart) and the user saw an
+  // unexplained gap on the dashboard. The in-card render branch below
+  // now paints a "no data in this range" empty state so the user
+  // understands the chart loaded but the selected window holds no
+  // measurements — distinct copy from the < 3-points "more days
+  // needed" hint so the two situations don't blur.
 
   const maxPointIndex = Math.max(0, (chartData?.length ?? 1) - 1);
 
@@ -1288,7 +1295,29 @@ export function HealthChart({
         <div className="flex h-48 items-center justify-center">
           <Loader2 className="text-primary h-6 w-6 animate-spin motion-reduce:animate-none" />
         </div>
-      ) : !chartData?.length ? null : (chartData?.length ?? 0) < 3 ? (
+      ) : !chartData?.length ? (
+        // v1.4.43 W11-M6 — empty-window state.
+        //
+        // Pre-fix the chart silently `return null`ed when `chartData`
+        // resolved empty (after-load, no points in the selected
+        // range). Paired with the < 3-points hint, the user's
+        // five visible chart states were:
+        //
+        //   - skeleton (loading)
+        //   - 1-2 daily points / many raw → "Mehr Messtage erforderlich"
+        //   - 1-2 daily points / < 3 raw  → "Erfasse mindestens 3 Einträge"
+        //   - ≥ 3 daily points            → line chart
+        //   - 0 daily points              → NOTHING
+        //
+        // The "nothing" branch read as a broken widget on the
+        // dashboard. Paint a distinct empty state explaining the
+        // selected range is empty so the user reaches for the range
+        // tabs or the quick-add instead of suspecting a bug.
+        <ChartEmptyState
+          title={t("charts.noDataInRangeTitle")}
+          description={t("charts.noDataInRangeDescription")}
+        />
+      ) : (chartData?.length ?? 0) < 3 ? (
         // v1.4.16 B1a — sparse-data placeholder. <3 daily points is too
         // few to render a meaningful trend; paint a friendly hint
         // instead so the dashboard doesn't look broken.