feat(ui): Listen-Einstellungen — Checkbox fuer serialize_on_file_overlap

Das Flag war bisher nur ueber MCP set_list_config erreichbar. UpdateListConfigDto
fuehrt es tri-state (null = gespeicherten Wert behalten), damit nur das
Listen-Modal es setzen/loeschen kann und kein anderer Aufrufer es per Omission
verliert.

src/ClaudeDo.Worker/External/ConfigMcpTools.cs

@@ -104,8 +104,8 @@ public sealed class ConfigMcpTools
 
         // Fields this tool doesn't expose but that live on the same row. They must survive every
         // write here — ListRepository.SetConfigAsync copies the entity verbatim, so anything left
-        // at its default would silently reset (SerializeOnFileOverlap in particular has no UI
-        // affordance at all, so a reset is invisible until tasks stop serializing).
+        // at its default would silently reset (a SerializeOnFileOverlap reset only shows up as
+        // tasks no longer serializing, long after this write).
         var hasUnrelatedSettings = existing is not null
             && (existing.SessionSkills is not null || existing.SerializeOnFileOverlap);