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/persons/[id]/+page.svelte

@@ -77,17 +77,21 @@ const coCorrespondents = $derived.by(() => {
 				/>
 			</div>
 
-			<PersonDocumentList
-				documents={sentDocuments}
-				heading={m.person_docs_heading()}
-				emptyMessage={m.person_no_docs()}
-			/>
+			<div class="mt-6">
+				<PersonDocumentList
+					documents={sentDocuments}
+					heading={m.person_docs_heading()}
+					emptyMessage={m.person_no_docs()}
+				/>
+			</div>
 
-			<PersonDocumentList
-				documents={receivedDocuments}
-				heading={m.person_received_docs_heading()}
-				emptyMessage={m.person_no_received_docs()}
-			/>
+			<div class="mt-6">
+				<PersonDocumentList
+					documents={receivedDocuments}
+					heading={m.person_received_docs_heading()}
+					emptyMessage={m.person_no_received_docs()}
+				/>
+			</div>
 		</div>
 	</div>
 </div>

frontend/src/routes/persons/[id]/PersonMergePanel.svelte

@@ -15,7 +15,7 @@ let mergeTargetId = $state('');
 let showMergeConfirm = $state(false);
 </script>
 
-<div class="mb-10 rounded-sm border border-red-200 bg-surface p-6 shadow-sm">
+<div class="mt-6 mb-10 rounded-sm border border-red-200 bg-surface p-6 shadow-sm">
 	<div>
 		<h2 class="mb-1 font-serif text-lg text-ink">{m.person_merge_heading()}</h2>
 		<p class="mb-5 font-sans text-sm text-ink-2">

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"
 	>