feat(stammbaum): show inferred relationship in the document drawer

- New presentational RelationshipBadge component (labelFromA → arrow →
  labelFromB) wired into DocumentMetadataDrawer's Personen column,
  rendered after the receivers block when both endpoints are family
  members.
- DocumentTopBar gains an optional inferredRelationship prop and
  passes it through.
- documents/[id]/+page.server.ts loads the badge: only when sender is
  a family member, exactly one receiver, and that receiver is also a
  family member; 404 (no path) → null.
- relationshipLabels.ts maps the backend label keys (parent/child/...)
  to localised strings, so the server load returns badge-ready strings.

Refs #358.

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

frontend/src/lib/relationshipLabels.ts

@@ -0,0 +1,44 @@
+import * as m from '$lib/paraglide/messages.js';
+
+/**
+ * Maps a backend inferred-label key (parent, uncle_aunt, ...) to its
+ * localised string. Unknown keys fall back to "distant".
+ */
+export function inferredRelationshipLabel(key: string): string {
+	switch (key) {
+		case 'parent':
+			return m.relation_inferred_parent();
+		case 'child':
+			return m.relation_inferred_child();
+		case 'spouse':
+			return m.relation_inferred_spouse();
+		case 'sibling':
+			return m.relation_inferred_sibling();
+		case 'grandparent':
+			return m.relation_inferred_grandparent();
+		case 'grandchild':
+			return m.relation_inferred_grandchild();
+		case 'great_grandparent':
+			return m.relation_inferred_great_grandparent();
+		case 'great_grandchild':
+			return m.relation_inferred_great_grandchild();
+		case 'uncle_aunt':
+			return m.relation_inferred_uncle_aunt();
+		case 'niece_nephew':
+			return m.relation_inferred_niece_nephew();
+		case 'great_uncle_aunt':
+			return m.relation_inferred_great_uncle_aunt();
+		case 'great_niece_nephew':
+			return m.relation_inferred_great_niece_nephew();
+		case 'inlaw_parent':
+			return m.relation_inferred_inlaw_parent();
+		case 'inlaw_child':
+			return m.relation_inferred_inlaw_child();
+		case 'sibling_inlaw':
+			return m.relation_inferred_sibling_inlaw();
+		case 'cousin_1':
+			return m.relation_inferred_cousin_1();
+		default:
+			return m.relation_inferred_distant();
+	}
+}