fix(loyalty): disable confirm button until category selected

PIN modal confirm button stays disabled when categories exist and
the action is stamp/earn but no category is selected yet.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/loyalty/templates/loyalty/store/terminal.html

@@ -379,7 +379,7 @@
             {{ _('loyalty.common.cancel') }}
         </button>
         <button @click="submitTransaction()"
-                :disabled="pinDigits.length !== 4 || processing"
+                :disabled="pinDigits.length !== 4 || processing || (categories.length > 0 && (pendingAction === 'stamp' || pendingAction === 'earn') && selectedCategories.length === 0)"
                 class="px-4 py-2 text-sm font-medium text-white bg-purple-600 rounded-lg hover:bg-purple-700 disabled:opacity-50">
             <span x-show="processing" x-html="$icon('spinner', 'w-4 h-4 mr-2 inline animate-spin')"></span>
             <span x-text="processing ? '{{ _('loyalty.store.terminal.processing')|replace("'", "\\'") }}' : '{{ _('loyalty.store.terminal.confirm')|replace("'", "\\'") }}'"></span>