feat(stammbaum): centre-on-person control in the panel title row (#692)

Add an onCentre control to StammbaumSidePanel (title row, both desktop aside
and mobile sheet). The page drives a one-shot centreOnId so StammbaumTree
recentres the canvas on the focal node (US-PAN-005). Also tighten the panel
spec's deathYear fixture to a valid type.

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

frontend/src/routes/stammbaum/+page.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-import { untrack } from 'svelte';
+import { untrack, tick } from 'svelte';
 import { m } from '$lib/paraglide/messages.js';
 import { page } from '$app/state';
 import { replaceState } from '$app/navigation';
@@ -42,6 +42,16 @@ function zoomIn() {
 function zoomOut() {
 	view = { ...view, z: clampZoom(view.z - ZOOM_STEP_KB) };
 }
+// One-shot recentre trigger: set the focal id, let StammbaumTree's effect read
+// it and emit the recentred view, then clear so the same person can be
+// re-centred on a later click (US-PAN-005).
+let centreOnId = $state<string | null>(null);
+async function centreOnSelected() {
+	centreOnId = selectedId;
+	await tick();
+	centreOnId = null;
+}
+
 let cancelAnimation = () => {};
 function fitToScreen() {
 	cancelAnimation();
@@ -112,6 +122,7 @@ $effect(() => {
 					edges={data.edges}
 					selectedId={selectedId}
 					panZoom={view}
+					centreOnId={centreOnId}
 					onPanZoom={(v) => (view = v)}
 					onSelect={(id) => (selectedId = id)}
 				/>
@@ -126,6 +137,7 @@ $effect(() => {
 						node={selectedNode}
 						canWrite={canWrite}
 						onClose={() => (selectedId = null)}
+						onCentre={centreOnSelected}
 					/>
 				</aside>
 				<!-- Mobile: dismissible bottom sheet (overlay, preserves pan/zoom) -->
@@ -133,6 +145,7 @@ $effect(() => {
 					node={selectedNode}
 					canWrite={canWrite}
 					onClose={() => (selectedId = null)}
+					onCentre={centreOnSelected}
 				/>
 			{/if}
 		</div>