fix(stammbaum): make gutter visibility prop-overridable for tests (#689)

CI kept failing on the two gutter-render tests because the vitest-browser
iframe viewport is narrower than 768 px → window.matchMedia(min-width:
768px) returns false → gutter is hidden → g[role="text"] selector
returns []. The previous synchronous-seed fix was insufficient because
matchMedia itself was the false branch.

Add an optional `showGutter?: boolean` prop. When set, it bypasses the
matchMedia detection — tests pass `showGutter: true` to assert the
rendered gutter, and `showGutter: false` to assert the absent path.
Production callers leave it undefined so the existing media-query
detection still governs visibility.

Refs #689

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/StammbaumTree.svelte

@@ -18,9 +18,16 @@ interface Props {
 	selectedId: string | null;
 	zoom: number;
 	onSelect: (id: string) => void;
+	/**
+	 * Force-show or force-hide the generation gutter. When undefined, falls
+	 * back to a `window.matchMedia('(min-width: 768px)')` detection so the
+	 * gutter only appears on md+ viewports. Tests pass an explicit boolean
+	 * to avoid depending on the vitest-browser iframe viewport.
+	 */
+	showGutter?: boolean;
 }
 
-let { nodes, edges, selectedId, zoom, onSelect }: Props = $props();
+let { nodes, edges, selectedId, zoom, onSelect, showGutter }: Props = $props();
 
 const layout = $derived.by<Layout>(() => buildLayout(nodes, edges));
 
@@ -39,13 +46,15 @@ let isMdOrUp = $state(
 		: false
 );
 $effect(() => {
+	if (showGutter !== undefined) return;
 	if (typeof window === 'undefined' || typeof window.matchMedia !== 'function') return;
 	const mq = window.matchMedia(GUTTER_MEDIA_QUERY);
 	const handler = (e: MediaQueryListEvent) => (isMdOrUp = e.matches);
 	mq.addEventListener('change', handler);
 	return () => mq.removeEventListener('change', handler);
 });
-const gutterWidth = $derived(isMdOrUp ? GUTTER_WIDTH_DESKTOP : 0);
+const gutterVisible = $derived(showGutter ?? isMdOrUp);
+const gutterWidth = $derived(gutterVisible ? GUTTER_WIDTH_DESKTOP : 0);
 
 type GutterRow = { rank: number; y: number; label: number | null };
 const gutterRows = $derived.by<GutterRow[]>(() => {