feat(stammbaum): land a fresh visit on the tree's top-left corner (#692)

At z=3 a pan of {0,0} centres on the tree midpoint; a fresh visit (no shared
?z) now anchors the viewBox to the tree's top-left corner via topLeftView
(the negative clamp limit), emitted on mount. Shared links still win.
Verified live: lands at cx<0, cy<0.

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

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

@@ -1,5 +1,5 @@
 <script lang="ts">
-import { untrack } from 'svelte';
+import { untrack, onMount } from 'svelte';
 import { SvelteMap, SvelteSet } from 'svelte/reactivity';
 import type { components } from '$lib/generated/api';
 import {
@@ -13,6 +13,7 @@ import {
 	type PanZoomState,
 	clampZoom,
 	recentreOn,
+	topLeftView,
 	ZOOM_STEP_KB
 } from '$lib/person/genealogy/panZoom';
 import { panZoomGestures } from '$lib/person/genealogy/panZoomGestures';
@@ -32,6 +33,8 @@ interface Props {
 	centreOnId?: string | null;
 	/** Fired on the first pointer interaction with the canvas (affordance dismiss). */
 	onActivity?: () => void;
+	/** When true, the initial view is anchored to the tree's top-left corner. */
+	anchorTopLeft?: boolean;
 	onSelect: (id: string) => void;
 	/**
 	 * Force-show or force-hide the generation gutter. When undefined, falls
@@ -50,6 +53,7 @@ let {
 	onPanZoom = () => {},
 	centreOnId = null,
 	onActivity,
+	anchorTopLeft = false,
 	onSelect,
 	showGutter
 }: Props = $props();
@@ -139,6 +143,12 @@ const railRows = $derived(
 		.map((r) => ({ rank: r.rank, label: r.label, centerY: r.y + NODE_H / 2 }))
 );
 
+// A fresh visit (no shared URL state) lands on the tree's top-left corner rather
+// than its centre (#692). Runs once after layout is available.
+onMount(() => {
+	if (anchorTopLeft) onPanZoom(topLeftView(baseDims.w, baseDims.h, panZoom.z));
+});
+
 const viewBox = $derived.by(() => {
 	const w = baseDims.w / panZoom.z;
 	const h = baseDims.h / panZoom.z;