fix(timeline): keep "No entries" for a bucket with nothing in it

A bucket that holds a mean but no lane entry is not empty; the means
say what it holds. The bar opened on such a bucket by default and called
it empty beside its own values.

src/components/timeline/__tests__/timeline-components.test.tsx

@@ -320,6 +320,15 @@ describe("selection bar head and hint", () => {
     expect(text(html)).toContain("Erkältung · 31. Dez. bis 8. Jan.");
   });
 
+  it("calls a bucket empty only when it holds neither an entry nor a value", () => {
+    // July 2026: no entry, but a systolic mean.
+    expect(bar("2026-07-15")).not.toContain("timeline-selection-empty");
+    // May 2026: no entry, no value.
+    expect(text(bar("2026-05-10"))).toContain(
+      "Keine Einträge in diesem Zeitraum",
+    );
+  });
+
   it("says how to pick a day only until one is picked", () => {
     expect(bar("2026-01-03", fullTimeline(), true)).toContain(
       'data-slot="timeline-selection-hint"',

src/components/timeline/selection-bar.tsx

@@ -73,6 +73,9 @@ export function SelectionBar({
   const periodTo = dayKey(dayNumber(bucketAfter(periodFrom, bucket)) - 1);
   const entries = itemsInPeriod(timeline.lanes, periodFrom, periodTo, today);
   const means = bucketValues(timeline.series, timeline.bucket, selected);
+  // A bucket with a value but no entry is not empty: the means below say
+  // what it holds, so "No entries" is kept for a stretch with nothing.
+  const hasValue = means.values.some((v) => v.mean !== null);
   // "Ø {values}": the words around the values, so the values can carry
   // their colour dots.
   const [meanBefore, meanAfter = ""] = t("timeline.selection.mean", {
@@ -109,7 +112,7 @@ export function SelectionBar({
       >
         {bucketTitle(periodFrom, bucket, intl, t)}
       </span>
-      {entries.length === 0 && (
+      {entries.length === 0 && !hasValue && (
         <span
           className="text-muted-foreground text-xs"
           data-slot="timeline-selection-empty"