feat(stammbaum): recentre on a node via centreOnId prop (#692)

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

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

@@ -1,4 +1,5 @@
 <script lang="ts">
+import { untrack } from 'svelte';
 import { SvelteMap, SvelteSet } from 'svelte/reactivity';
 import type { components } from '$lib/generated/api';
 import {
@@ -8,7 +9,12 @@ import {
 	ROW_GAP,
 	type Layout
 } from '$lib/person/genealogy/layout/buildLayout';
-import { type PanZoomState, clampZoom, ZOOM_STEP_KB } from '$lib/person/genealogy/panZoom';
+import {
+	type PanZoomState,
+	clampZoom,
+	recentreOn,
+	ZOOM_STEP_KB
+} from '$lib/person/genealogy/panZoom';
 import { panZoomGestures } from '$lib/person/genealogy/panZoomGestures';
 
 type PersonNodeDTO = components['schemas']['PersonNodeDTO'];
@@ -21,6 +27,8 @@ interface Props {
 	panZoom: PanZoomState;
 	/** Emitted when the keyboard, a gesture, or a recentre changes the view. */
 	onPanZoom?: (state: PanZoomState) => void;
+	/** When set to a node id, the canvas recentres on that node (US-PAN-005). */
+	centreOnId?: string | null;
 	onSelect: (id: string) => void;
 	/**
 	 * Force-show or force-hide the generation gutter. When undefined, falls
@@ -37,6 +45,7 @@ let {
 	selectedId,
 	panZoom,
 	onPanZoom = () => {},
+	centreOnId = null,
 	onSelect,
 	showGutter
 }: Props = $props();
@@ -130,6 +139,18 @@ function nodeCenter(id: string): { x: number; y: number } | null {
 	return { x: p.x + NODE_W / 2, y: p.y + NODE_H / 2 };
 }
 
+// Recentre when the parent sets centreOnId (US-PAN-005). Only centreOnId is a
+// tracked dependency — the current view is read untracked so a normal pan does
+// not retrigger a recentre.
+$effect(() => {
+	const id = centreOnId;
+	if (!id) return;
+	untrack(() => {
+		const c = nodeCenter(id);
+		if (c) onPanZoom(recentreOn(c, baseCentre, panZoom, true));
+	});
+});
+
 let focusedId = $state<string | null>(null);
 
 function handleNodeKey(event: KeyboardEvent, id: string) {