refactor(geschichte): extract PersonNameFormatter to eliminate duplicated name-join logic

Create PersonNameFormatter with a single static join(firstName, lastName) method.
Replace the inline string concatenation in GeschichteService.toView() and the
private join() method in JourneyItemService with calls to PersonNameFormatter.join().
The new helper handles null-safety and trimming consistently in one place.

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

backend/src/main/java/org/raddatz/familienarchiv/geschichte/GeschichteService.java

@@ -71,8 +71,7 @@ public class GeschichteService {
         AppUser author = g.getAuthor();
         GeschichteView.AuthorView authorView = null;
         if (author != null) {
-            String displayName = ((author.getFirstName() != null ? author.getFirstName() : "")
+            String displayName = PersonNameFormatter.join(author.getFirstName(), author.getLastName());
-                    + " " + (author.getLastName() != null ? author.getLastName() : "")).trim();
             if (displayName.isBlank()) displayName = "[Unbekannt]";
             authorView = new GeschichteView.AuthorView(author.getId(), displayName);
         }

backend/src/main/java/org/raddatz/familienarchiv/geschichte/PersonNameFormatter.java

@@ -0,0 +1,22 @@
+package org.raddatz.familienarchiv.geschichte;
+
+/**
+ * Utility for joining a person's first and last name into a display string.
+ * Centralises the logic that was previously duplicated across GeschichteService
+ * and JourneyItemService.
+ */
+public class PersonNameFormatter {
+
+    private PersonNameFormatter() {
+        // utility class — no instances
+    }
+
+    public static String join(String firstName, String lastName) {
+        String first = firstName != null ? firstName.trim() : "";
+        String last = lastName != null ? lastName.trim() : "";
+        if (first.isEmpty() && last.isEmpty()) return "";
+        if (first.isEmpty()) return last;
+        if (last.isEmpty()) return first;
+        return first + " " + last;
+    }
+}

backend/src/main/java/org/raddatz/familienarchiv/geschichte/journeyitem/JourneyItemService.java

@@ -13,6 +13,7 @@ import org.raddatz.familienarchiv.geschichte.Geschichte;
 import org.raddatz.familienarchiv.geschichte.GeschichteQueryService;
 import org.raddatz.familienarchiv.geschichte.GeschichteType;
 import org.raddatz.familienarchiv.geschichte.DocumentSummary;
+import org.raddatz.familienarchiv.geschichte.PersonNameFormatter;
 import org.raddatz.familienarchiv.person.Person;
 import org.raddatz.familienarchiv.user.AppUser;
 import org.raddatz.familienarchiv.user.UserService;
@@ -216,7 +217,7 @@ public class JourneyItemService {
     private static String buildSenderName(Document doc) {
         Person sender = doc.getSender();
         if (sender != null) {
-            String name = join(sender.getFirstName(), sender.getLastName());
+            String name = PersonNameFormatter.join(sender.getFirstName(), sender.getLastName());
             if (!name.isBlank()) return name;
         }
         String senderText = doc.getSenderText();
@@ -228,7 +229,7 @@ public class JourneyItemService {
         return receivers.stream()
                 .min(Comparator.comparing(p -> sortKey(p.getLastName()) + " " + sortKey(p.getFirstName())))
                 .map(p -> {
-                    String name = join(p.getFirstName(), p.getLastName());
+                    String name = PersonNameFormatter.join(p.getFirstName(), p.getLastName());
                     return name.isBlank() ? null : name;
                 })
                 .orElse(null);
@@ -239,10 +240,6 @@ public class JourneyItemService {
         return raw.trim();
     }
 
-    private static String join(String first, String last) {
-        return ((first != null ? first : "") + " " + (last != null ? last : "")).trim();
-    }
-
     private static String sortKey(String s) {
         return s != null ? s : "";
     }