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

@@ -8,14 +8,11 @@ import type { PageData } from './$types';
 
 let { data }: { data: PageData } = $props();
 
-const meta = $derived(timelineMeta(data.timeline));
 const hasContent = $derived(data.timeline.years.length > 0 || data.timeline.undated.length > 0);
 
 // Layer-filter state (#780). Layer hiding is client-side only — the whole
 // timeline is loaded once by #779's SSR load and we derive a filtered view of
-// it here; there is no goto, no URL param, and no extra fetch. Known limitation
-// (D1): the meta-line counts above stay on the unfiltered timeline, so they
-// include entries the active toggles hide.
+// it here; there is no goto, no URL param, and no extra fetch.
 let personalOn = $state(true);
 let historicalOn = $state(true);
 let lettersOn = $state(true);
@@ -27,6 +24,11 @@ const filteredEmpty = $derived(
 	filteredTimeline.years.length === 0 && filteredTimeline.undated.length === 0
 );
 
+// Meta-line figures track the *filtered* view, so the header counts always
+// match what is actually on screen once layers are toggled off (#780 — this
+// closes the prior D1 limitation, where the counts stayed on the full timeline).
+const meta = $derived(timelineMeta(filteredTimeline));
+
 function resetFilters() {
 	personalOn = true;
 	historicalOn = true;