feat(stammbaum): drive viewBox from PanZoomState (pan + zoom) (#692)

Replace the scalar zoom prop with a {x,y,z} PanZoomState. The viewBox centre
is offset by the pan and width/height scaled by zoom; the default {0,0,1}
frames the whole tree (fit-to-screen). Page header buttons now step view.z
through clampZoom over the resolved 0.25–3.0 range.

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

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

@@ -3,6 +3,12 @@ import { m } from '$lib/paraglide/messages.js';
 import { page } from '$app/state';
 import StammbaumTree from '$lib/person/genealogy/StammbaumTree.svelte';
 import StammbaumSidePanel from '$lib/person/genealogy/StammbaumSidePanel.svelte';
+import {
+	type PanZoomState,
+	DEFAULT_VIEW,
+	clampZoom,
+	ZOOM_STEP_KB
+} from '$lib/person/genealogy/panZoom';
 import type { components } from '$lib/generated/api';
 
 type PersonNodeDTO = components['schemas']['PersonNodeDTO'];
@@ -23,12 +29,12 @@ let selectedId = $state<string | null>(
 
 const selectedNode = $derived(data.nodes.find((n) => n.id === selectedId) ?? null);
 
-let zoom = $state(1);
+let view = $state<PanZoomState>(DEFAULT_VIEW);
 function zoomIn() {
-	zoom = Math.min(2, zoom + 0.1);
+	view = { ...view, z: clampZoom(view.z + ZOOM_STEP_KB) };
 }
 function zoomOut() {
-	zoom = Math.max(0.4, zoom - 0.1);
+	view = { ...view, z: clampZoom(view.z - ZOOM_STEP_KB) };
 }
 </script>
 
@@ -97,7 +103,7 @@ function zoomOut() {
 					nodes={data.nodes}
 					edges={data.edges}
 					selectedId={selectedId}
-					zoom={zoom}
+					panZoom={view}
 					onSelect={(id) => (selectedId = id)}
 				/>
 			</div>