fix(timeline): track the meta-line counts to the filtered view

The /zeitstrahl header sub-line counted the unfiltered timeline, so a
hidden layer (e.g. Letters off) still showed its entries in the totals
("1 Brief" with no letters on screen) — the documented D1 limitation.
Derive the meta from filteredTimeline so the range and letter/event
counts always match what is actually rendered. hasContent stays on the
full timeline so the filter bar and meta line still appear whenever the
archive has content.

Refs #780

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/zeitstrahl/page.svelte.spec.ts

@@ -196,4 +196,20 @@ describe('/zeitstrahl layer filter (#780)', () => {
 		await page.getByTestId('timeline-filter-empty-reset').click();
 		await expect.element(page.getByText('Brief Eins')).toBeVisible();
 	});
+
+	it('recomputes the meta-line counts from the filtered view, so a hidden layer drops out of the totals (#780, resolves D1)', async () => {
+		render(Page, { data: pageData(mixed()) });
+		const meta = page.getByTestId('timeline-meta');
+		// all layers on → the one letter and the two events are counted
+		await expect.element(meta).toHaveTextContent(m.timeline_letters_count_singular());
+		await expect.element(meta).toHaveTextContent(m.timeline_events_count({ count: 2 }));
+
+		await openBar();
+		await page.getByTestId('timeline-filter-letters').click();
+
+		// the hidden letter leaves the count instead of lingering as "1 Brief";
+		// the event total is untouched
+		await expect.element(meta).not.toHaveTextContent(m.timeline_letters_count_singular());
+		await expect.element(meta).toHaveTextContent(m.timeline_events_count({ count: 2 }));
+	});
 });