Planner: uniquePlanName beim Kopieren anwenden

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

js/planner.js

@@ -81,7 +81,7 @@ function copyPlan(id) {
     const copy = {
         ...JSON.parse(JSON.stringify(orig)),
         id:        crypto.randomUUID(),
-        name:      orig.name + ' (Kopie)',
+        name:      uniquePlanName(orig.name + ' (Kopie)'),
         createdAt: Date.now(),
         updatedAt: Date.now()
     };