Fix SwearBrotherhood placeholder not being replaced

Use string concatenation instead of interpolated string for the template
to ensure {NewBrother} and {FactionHead} placeholders are correctly
replaced by DynamicTextNotification.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/main/csharp/net/eagle0/clients/unity/eagle0/Assets/Eagle/Notifications/ARNNotifications/SwearBrotherhoodDetailsNotificationGenerator.cs

@@ -17,8 +17,8 @@ namespace eagle.Notifications.ARNNotifications {
 
             var affectedProvinces = currentModel.ProvincesForFaction(details.FactionId);
 
-            string textTemplate =
-                    $"{{NewBrother}} has sworn brotherhood with {factionName}'s leader {{FactionHead}}.\n\n";
+            string textTemplate = "{NewBrother} has sworn brotherhood with " + factionName +
+                                  "'s leader {FactionHead}.\n\n";
 
             var heroPlaceholders = new Dictionary<string, (string nameTextId, string fallback)> {
                 { "NewBrother", (newBrother.NameTextId, "A hero") },