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/routes/stammbaum/+page.svelte

@@ -140,6 +140,7 @@ $effect(() => {
 					selectedId={selectedId}
 					panZoom={view}
 					centreOnId={centreOnId}
+					anchorTopLeft={!page.url.searchParams.has('z')}
 					onPanZoom={(v) => (view = v)}
 					onActivity={() => (canvasActivity = true)}
 					onSelect={(id) => (selectedId = id)}