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.test.ts

@@ -578,6 +578,38 @@ describe('StammbaumTree keyboard pan/zoom (#692)', () => {
 		expect(onSelect).not.toHaveBeenCalled();
 	});
 
+	it('recentres on a node when centreOnId is set, auto-zooming to legible (US-PAN-005)', async () => {
+		const onPanZoom = vi.fn();
+		render(StammbaumTree, {
+			nodes: [
+				{ id: ID_A, displayName: 'Anna', familyMember: true },
+				{ id: ID_B, displayName: 'Bertha', familyMember: true }
+			],
+			edges: [
+				{
+					id: 'sp',
+					personId: ID_A,
+					relatedPersonId: ID_B,
+					personDisplayName: 'Anna',
+					relatedPersonDisplayName: 'Bertha',
+					relationType: 'SPOUSE_OF'
+				}
+			],
+			selectedId: null,
+			panZoom: { x: 0, y: 0, z: 0.5 },
+			centreOnId: ID_A,
+			onPanZoom,
+			onSelect: () => {}
+		});
+
+		await vi.waitFor(() => expect(onPanZoom).toHaveBeenCalled());
+		const view = onPanZoom.mock.calls.at(-1)![0];
+		// Anna sits left of the two-node midpoint → pan x is negative.
+		expect(view.x).toBeLessThan(0);
+		// Zoomed out below legible → snapped up to 1.
+		expect(view.z).toBe(1);
+	});
+
 	it('does not call onSelect for other keys', async () => {
 		const onSelect = vi.fn();
 		render(StammbaumTree, {