fix(stammbaum): derived relationship names link to person page in StammbaumCard

The <span> in the derived-relationships list is replaced with <a href>
so keyboard and pointer users can navigate directly from the edit card,
consistent with PersonRelationshipsCard.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/StammbaumCard.svelte

@@ -157,9 +157,12 @@ function yearRange(rel: RelationshipDTO): string {
 						>
 							{inferredRelationshipLabel(derived.label)}
 						</span>
-						<span class="min-w-0 flex-1 truncate font-serif text-sm text-ink-2">
+						<a
+							href="/persons/{derived.person.id}"
+							class="min-w-0 flex-1 truncate font-serif text-sm text-ink-2 hover:underline"
+						>
 							{derived.person.displayName}
-						</span>
+						</a>
 					</li>
 				{/each}
 			</ul>