fix(loyalty-admin): existing-program warning crashes when no merchant selected

The warning panel was wrapped in x-show, which only toggles
display:none — child :href bindings still evaluate, so the
'selectedMerchant.id' access inside the link threw
'TypeError: can't access property "id", selectedMerchant is null'
on every reactive update before a merchant was picked.

Switch to <template x-if> so the element is removed from the DOM
entirely when the condition is false; child bindings then never
run.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/modules/loyalty/templates/loyalty/admin/programs.html

@@ -310,8 +310,8 @@
     </div>
 
     <!-- Existing program warning -->
-    <div x-show="selectedMerchant && existingProgramForMerchant(selectedMerchant.id)"
-         class="mb-4 px-4 py-3 bg-yellow-50 border border-yellow-200 rounded-lg dark:bg-yellow-900/20 dark:border-yellow-800">
+    <template x-if="selectedMerchant && existingProgramForMerchant(selectedMerchant.id)">
+        <div class="mb-4 px-4 py-3 bg-yellow-50 border border-yellow-200 rounded-lg dark:bg-yellow-900/20 dark:border-yellow-800">
             <div class="flex items-start">
                 <span x-html="$icon('exclamation-triangle', 'w-5 h-5 text-yellow-500 mr-3 mt-0.5 flex-shrink-0')"></span>
                 <div>
@@ -324,6 +324,7 @@
                 </div>
             </div>
         </div>
+    </template>
 
     <!-- Actions -->
     <div class="flex justify-end gap-3">