style(stammbaum): tighten vertical rhythm around relationship cards

- /stammbaum: drop the global py-6 top gap so the page header butts
  up against the navbar, matching its full-bleed canvas layout
- person detail: add mt-6 around the document lists so they don't
  sit flush against the Beziehungen card
- person edit: add mt-6 to PersonMergePanel so the merge box doesn't
  collide with the StammbaumCard above it

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -34,7 +34,7 @@ function zoomOut() {
 }
 </script>
 
-<div class="flex h-full flex-col">
+<div class="-mt-6 flex h-full flex-col">
 	<header
 		class="flex shrink-0 items-center justify-between border-b border-line bg-surface px-6 py-4"
 	>