fix(stammbaum): i18n the StammbaumCard heading (de/en/es)

Hardcoded 'Stammbaum & Beziehungen' heading replaced with
m.stammbaum_relationships_heading(); new key added to all
three message files.

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

frontend/src/lib/components/StammbaumCard.svelte

@@ -67,7 +67,7 @@ function yearRange(rel: RelationshipDTO): string {
 	<!-- Header row: heading + family-member toggle -->
 	<div class="mb-5 flex items-start justify-between gap-4">
 		<h2 class="text-xs font-bold tracking-widest text-ink-3 uppercase">
-			Stammbaum &amp; Beziehungen
+			{m.stammbaum_relationships_heading()}
 		</h2>
 		{#if canWrite}
 			<form method="POST" action="?/toggleFamilyMember" use:enhance>