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>